Th1 Effector Responses Research Articles

Staphylococcal Enterotoxin B In Vivo Modulates both Gamma Interferon Receptor Expression and Ligand-Induced Activation of Signal Transducer and Activator of Transcription 1 in T Cells

Superantigens (SAg) are bacterial exotoxins that provoke extreme responses in the immune system; for example, the acute hyperactivation of SAg-reactive T cells that leads to toxic shock syndrome is followed within days by strong immunosuppression. The gamma interferon (IFN-gamma) response is deeply affected in both extremes. The implication of IFN-gamma in the pathophysiology of lethal shock induced in mice after a secondary challenge with the SAg staphylococcal enterotoxin B (SEB) prompted us to study the regulation of IFN-gamma secretion and the intracellular response. We demonstrate in this study that a rechallenge with SEB becomes lethal only when given inside a critical time window after SEB priming and is associated with an increase of IFN-gamma serum release 72 h after priming. However, at this time, a selective blockade of IFN-gamma/STAT1 signaling develops in spleen cells, correlating with a lack of expression of the IFN-gamma receptor beta subunit and STAT1 in the T-cell population. Selective blockade of the STAT1 signaling pathway--while simultaneously maintaining STAT3 signaling and expression--may be a protective mechanism that shortens IFN-gamma production during the Th1 effector response. This blockade may also have consequences on switching towards a suppressor phenotype with chronic exposure to the superantigen.

HIV-1 transgenic rat CD4+ T cells develop decreased CD28 responsiveness and suboptimal Lck tyrosine dephosphorylation following activation

Impaired CD4+ T cell responses, resulting in dysregulated T-helper 1 (Th1) effector and memory responses, are a common result of HIV-1 infection. These defects are often preceded by decreased expression and function of the α/β T cell receptor (TCR)–CD3 complex and of co-stimulatory molecules including CD28, resulting in altered T cell proliferation, cytokine secretion and cell survival. We have previously shown that HIV Tg rats have defective development of T cell effector function and generation of specific effector/memory T cell subsets. Here we identify abnormalities in activated HIV-1 Tg rat CD4+ T cells that include decreased pY505 dephosphorylation of Lck (required for Lck activation), decreased CD28 function, reduced expression of the anti-apoptotic molecule Bcl-xL, decreased secretion of the mitogenic lympokine interleukin-2 (IL-2) and increased activation induced apoptosis. These events likely lead to defects in antigen-specific signaling and may help explain the disruption of Th1 responses and the generation of specific effector/memory subsets in transgenic CD4+ T cells.

Induction of chronic colitis in IL-10 deficient mice requires IL-4

Th 1 cells activated by IL-12 and secreting IFN-γ have been described as the main mediators for onset and maintenance of chronic colitis in IL-10 deficient mice. It was therefore surprising that mice deficient for IL-4 in addition to IL-10 showed intestinal pathology very rarely, whereas IL-10 KO mice developed rectal prolapse in most cases. To investigate the underlying mechanisms, we studied changes of ongoing inflammatory processes in mice deficient for IL-4, IL-10 or both cytokines. Levels of IFN-γ, IL-12p40 and MHCII mRNA were elevated to a much higher degree in colonic tissue of IL-10 KO compared to IL-4/10 KO at the onset of colitis. Furthermore, the influx of eosinophils, a marker for Th2 responses, was investigated. Only IL-10 deficient mice displayed a significant increase of eosinophils in the lamina propria of the colon and rectum. In contrast, IL-4/10 deficient mice had eosinophil levels comparable to wildtype controls and IL-4 KO. Together these results indicate an important role of IL-4 for the onset of colitis in IL-10 KO mice by promoting a Th1 response and induction of a deleterious Th2 effector response.

Biologic and Novel Therapy of the Inflammatory Bowel Diseases

IBD is now best considered as a group of genetic disorders of immune dysregulation with failure of downregulation of the intestinal mucosas normal immune response to its intraluminal commensal bacterial milieu combined with inappropriate migration of antigenic material triggered by multiple host (appendectomy, pregnancy, breast feeding, age) and environmental factors (tobacco smoking, birth control pill, NSAIDS). The result is the predominance of an effector or pro-inflammatory cytokine response due to clonal expansion of T1 and T2 helper lymphocytes (Th1 and Th2 response) combined with an inadequate regulatory cytokine response by T3 helper (Th 3) and T1 regulatory (Tr1) lymphocytes. The persistence of this imbalance in the inflammatory response then leads to tissue damage resulting in both local and systemic illness. In human IBD, a distinction can now be drawn between Crohns disease with its Th1 effector response characterized by high levels of the cytokines interleukin-12 (IL-12), interferon gamma (IFNγ) and tumor necrosis factor (TNF) leading to transmural inflammation with granulomas, and ulcerative colitis with a Th2 effector response, high levels of interleukin-4 (IL-4), interleukin-5 (IL-5), interleukin-6 (IL-6), and interleukin-13 (IL-13) characterized by strictly mucosal inflammation. The difficulty with adherence to current maintenance therapy for the inflammatory bowel diseases as well as the toxicity and relative ineffectiveness of induction therapy has provided the impetus for the development of new biologic and novel therapies which will be reviewed in this article. Biologic therapy has been divided into five areas of study: 1) native biologic preparations and isolates such as vaccines, hormone fragments and blood products, 2) recombinant peptides or proteins, including granulocyte macrophage colony stimulating factor, 3) monoclonal antibody-based therapies, 4) anti-sense nucleotides to nucleic acids, and 5) cell and gene therapies. Since the early 1990s, investigation in all five of these areas has yielded numerous potential agents for use in treating IBD more effectively and safely. While only one, infliximab (Remicade), is FDA-approved, several additional agents should be available for clinical use in the near future with the promise of more to follow. Our discussion of biologic and novel therapies represents a review of the most promising treatments and a status report of on-going clinical trials. Peripheral leukocyte apheresis and stem cell transplants, while suggesting an exciting new pathway for investigation, will not be reviewed. Our discussion is organized by each agents hypothesized mode of intervention in altering the pathogenesis of IBD. Keywords: infliximab, crohns disease (cd), ulcerative colitis (uc), corticosteroids, ileal pouch anal anastomosis (ipaa), tumor necrosis factor (tnf), anti-sense nucleotides, stem cell transplants, (gm-csf), anti-tnf agents

The growing complexity of TGF‐β as a T‐cell regulator

When Mosmann et al. first classified mouse CD4 T-cell clones according to the cytokines that they produced1 and showed that these subsets possessed different effector functions2 a new era in our understanding of immunology had begun. Since then, substantial effort has gone into understanding how T-cell subsets function in health and disease, the factors that govern their differentiation and more recently the characterization and function of regulatory T cells that control immune responses. Transforming growth factor-beta (TGF-β) is a pleiotropic cytokine that regulates diverse biological processes during development and homeostasis. In this issue of Immunology Smeltz et al. describe an unexpected effect of this cytokine on CD4 T-cell differentiation and IFN-γ synthesis.3 Extensive studies on T-cell subsets revealed that T helper 1 (Th1) and T helper 2 (Th2) cells were mutually exclusive and that IFN-γ, the signature cytokine of Th1 cells, inhibited the growth of Th2 cells4 while IL-4, the cytokine that defines Th2 cells, was inhibitory for Th1 cells5. Furthermore, both of these subsets produced cytokines that served as autocrine growth factors—IL-2 for Th1 and IL-4 for Th2 cells6,7. Historically IFN-γ was the first cytokine to be identified that promoted Th1 responses4. IFN-γ selectively induced naive CD45RA, but not memory CD45RO, T cells to secrete IFN-γ8 and express the Th1-specific transcription factor T-bet9. IL-4 appeared to be essential for Th2 development7. A further level of control was introduced by cytokines produced by antigen presenting cells (APC). APC derived IL-1210,11 and IL-1812 work in synergy to promote Th1 cell differentiation. Interestingly, comparable Th2 promoting cytokines from APC have not been described but APC that are low IL-12 producers, as in the lung, appear to preferentially induce Th2 cells13. The IL-4 that is necessary for Th2 development may be derived from a subset of natural killer (NK) cells14 or from the CD4 T cells themselves15. In addition to extracellular cytokines, the nature of the T-cell receptor stimulus and the presence, or absence, of a costimulus also regulate T-cell differentiation: high affinity T-cell receptor–MHC peptide interactions leading to a Th1 response while low affinity lead to a Th2 cytokine profile16–19. To control aberrant immune responses there are, in addition, regulatory T cells. These include T regulatory 1 (Tr1) cells that make IL-10 which inhibits both Th120 and Th221 responses and T helper 3 cells (Th3) that promote IgA responses in the gastrointestinal tract via TGF-β22 and also inhibit Th1 and Th2 responses23,24. TGF-β has also been shown to inhibit GATA-3 expression and Th2 development9 as well as airway Th2 cell induced inflammation21. It has also been shown that CD4-dominant negative TGF-βRII mice are unable to mount effective Th1 responses to pathogens such as Leishmania25 and suffer impaired immunity to tumors26. In this issue of the journal, Smeltz et al. demonstrate that, under certain circumstances, TGF-β can actually promote Th1 cell differentiation.3 This runs counter to the prevailing vision of TGF-β as a Th1 as well as a Th2 inhibitory cytokine25. Smeltz et al. suggest that TGF-β mediates this effect by inhibiting IL-4, which is inhibitory for Th1 responses. Indeed, Lingnau et al. have shown that a combination of TGF-β and IL-4 induced IFN-γ production in human CD4 T cells stimulated with antiαβ T-cell receptor (TcR) specific antibody27. Consistent with other reports9, Smeltz et al. also report that TGF-β inhibited GATA-3 gene expression and IL-4 synthesis, but also found that TGF-β augmented the generation of IFN-γ producing cells.3 They showed and that this process was independent of IL-12 and signal transducer and activator of transcription-4 (STAT-4) signaling but was dependent on endogenous IFN-γ, itself an important inducer of T-bet28. One explanation for the different effects of TGF-β on Th1 cell differentiation observed by these investigators may be the way in which the Th1 cells were generated. Smeltz et al. stimulated transgenic D011.10 CD4 T cells with specific ovalbumin peptide with or without cytokines and re-stimulated after one week with anti-CD3.3 Other investigators have used anti-CD3/CD28 to mimic T-cell activation25. It is also known that TGF-β signaling is mediated via the intracellular signaling molecules SMAD 2, 3 and 429. Hence an alternative explanation for the different effects of TGF-β observed could be based on the cell-specific phosphorylation of SMADs or in the stoichiometry of the SMAD signaling complex. Whatever the explanation, these findings serve to remind us that T cells do not develop in isolation but receive a complex array of signals whose balance determines their differentiation into effector cells.

Cutaneous Antigen Priming via Gene Gun Leads to Skin-Selective Th2 Immune-Inflammatory Responses

It is becoming increasingly evident that the compartmentalization of immune responses is governed, in part, by tissue-selective homing instructions imprinted during T cell differentiation. In the context of allergic diseases, the fact that "disease" primarily manifests in particular tissue sites, despite pervasive allergen exposure, supports this notion. However, whether the original site of Ag exposure distinctly privileges memory Th2 immune-inflammatory responses to the same site, while sparing remote tissue compartments, remains to be fully investigated. We examined whether skin-targeted delivery of plasmid DNA encoding OVA via gene-gun technology in mice could generate allergic sensitization and give rise to Th2 effector responses in the skin as well as in the lung upon subsequent Ag encounter. Our data show that cutaneous Ag priming induced OVA-specific serum IgE and IgG1, robust Th2-cytokine production, and late-phase cutaneous responses and systemic anaphylactic shock upon skin and systemic Ag recall, respectively. However, repeated respiratory exposure to aerosolized OVA failed to instigate airway inflammatory responses in cutaneous Ag-primed mice, but not in mice initially sensitized to OVA via the respiratory mucosa. Importantly, these contrasting airway memory responses correlated with the occurrence of Th2 differentiation events at anatomically separate sites: indeed cutaneous Ag priming resulted in Ag-specific proliferative responses and Th2 differentiation in skin-, but not thoracic-, draining lymph nodes. These data indicate that Ag exposure to the skin leads to Th2 differentiation within skin-draining lymph nodes and subsequent Th2 immunity that is selectively manifested in the skin.

Cellular FLIP (Long Isoform) Overexpression in T Cells Drives Th2 Effector Responses and Promotes Immunoregulation in Experimental Autoimmune Encephalomyelitis

Cellular FLIP (c-FLIP) is an endogenous inhibitor of death receptor-induced apoptosis through the caspase 8 pathway. It is an NF-kappaB-inducible protein thought to promote the survival of T cells upon activation, and its down-regulation has been implicated in activation-induced cell death. We have generated transgenic mice overexpressing human c-FLIP long form (c-FLIP(L)) specifically in T cells using the CD2 promoter (TgFLIP(L)). TgFLIP(L) mice exhibit increased IgG1 production upon stimulation by a T cell-dependent Ag and a markedly enhanced contact hypersensitivity response to allergen. In addition to showing augmented Th2-type responses, TgFLIP(L) mice are resistant to the development of myelin oligodendrocyte glycoprotein 35-55 peptide-induced experimental autoimmune encephalomyelitis, a Th1-driven autoimmune disease. In vitro analyses revealed that T cells of TgFLIP(L) mice proliferate normally, but produce higher levels of IL-2 and show preferential maturation of Th2 cytokine-producing cells in response to antigenic stimulation. After adoptive transfer, these (Th2) cells protected wild-type recipient mice from experimental autoimmune encephalomyelitis induction. Our results show that the constitutive overexpression of c-FLIP(L) in T cells is sufficient to drive Th2 polarization of effector T cell responses and indicate that it might function as a key regulator of Th cell differentiation.

Human dendritic cells respond to Helicobacter pylori, promoting NK cell and Th1-effector responses in vitro.

Helicobacter pylori infection leads to chronic gastric inflammation. The current study determined the response of human APCs, NK cells, and T cells toward the bacteria in vitro. Human monocyte-derived dendritic cells (DC) were incubated with bacteria for 48 h. Intact H. pylori at a multitude of infection 5 stimulated the expression of MHC class II (4- to 7-fold), CD80, and CD86 B7 molecules (10- to 12-fold) and the CD83 costimulatory molecule (>30-fold) as well as IL-12 secretion (>50-fold) in DCs, and thereby, strongly induced their maturation and activation. CD56(+)/CD4(-) NK cells, as well as CD4(+)/CD45RA(+) naive T cells, were isolated and incubated with DCs pulsed with intact bacteria or different cellular fractions. Coculture of H. pylori-pulsed DCs with NK cells strongly potentiated the secretion of TNF-alpha and IFN-gamma. Coculture of naive T cells with H. pylori-pulsed DCs significantly enhanced TNF-alpha, IFN-gamma, and IL-2 secretion as well as T-bet mRNA levels, while GATA-3 mRNA was lowered. However, the effect appeared attenuated compared with coculture with Escherichia coli. A greater stimulation was seen with naive T cells and DCs pulsed with H. pylori membrane preparations. Intact H. pylori potently induced the maturation and activation of human monocyte-derived DC and thereby promote NK and Th1 effector responses. The strong activation of NK cells may be important for the innate immune response. Th1-polarized T cells were induced especially by incubation with membrane preparations of H. pylori, suggesting that membrane proteins may account for the specific adaptive immune response.

Essential role of dendritic cell CD80/CD86 costimulation in the induction, but not reactivation, of TH2 effector responses in a mouse model of asthma

BackgroundAirway dendritic cells (DCs) are crucial for the generation of TH2 cells from naive T cells during sensitization and for reactivation of primed TH2 cells on allergen challenge in mouse models of asthma. It is unknown whether CD80/CD86 costimulation is necessary during both phases of the response because primed T cells rely less on costimulatory molecules compared with naive T cells. ObjectiveWe sought to study the contribution of CD80/CD86 costimulatory molecules on DCs during sensitization or challenge in a mouse model of asthma. MethodsNaive BALB/c mice received an intratracheal injection of ovalbumin (OVA)–pulsed DCs obtained from the bone marrow of wild-type (WT) or CD80/CD86−/− mice and were subsequently challenged with OVA aerosol to address the role of costimulation during sensitization. OVA-sensitized mice received OVA-pulsed WT or CD80/CD86−/− DCs without OVA aerosol to address the role of costimulation during challenge. ResultsWT DCs induced the proliferation and effector TH2 differentiation of naive OVA-specific T cells, whereas CD80/CD86−/− DCs induced only proliferation. Not surprisingly, WT DCs but not CD80/CD86−/− DCs induced sensitization to OVA in naive mice. In contrast, in OVA-sensitized mice intratracheal injection of CD80/CD86−/− OVA-pulsed DCs led to eosinophilic airway inflammation, goblet cell hyperplasia, and effector TH2 cytokine production that was not different from that seen after injection with WT OVA-DCs, even when the inducible costimulator ICOS was blocked or cytotoxic T lymphocyte–associated antigen 4 immunoglobulin was given. ConclusionCD80/CD86 costimulation on DCs is only necessary during priming of naive T cells into TH2 cells but not during restimulation of previously primed TH2 cells in the challenge phase.

Intrinsic renal cell expression of CD40 directs Th1 effectors inducing experimental crescentic glomerulonephritis.

Evidence suggests that human and experimental crescentic GN results from Th1-predominant immunity to glomerular antigens. CD40/CD154 signaling plays a key role in initiating Th1 responses and may direct Th1 effector responses. The role of CD40 in the development of GN was assessed in murine experimental anti-glomerular basement membrane GN. In this model, C57BL/6 wild-type (WT) mice sensitized to sheep globulin develop crescentic GN resulting from Th1 effector responses when challenged with sheep globulin planted in glomeruli. CD40-/- mice do not develop immunity in response to sheep globulin and thus fail to develop effector responses or significant GN. CD40 is expressed in nephritic glomeruli, suggesting a potential role for intrarenal CD40-CD154 interactions in injurious effector responses. Immune neutralization of the CD40 ligand (CD154) at the time of challenge significantly reduced accumulation of Th1 effectors and injury. The role of CD40 expression by renal cells was assessed by comparing GN in WT-->CD40-/- chimeras (absent renal but intact bone marrow CD40) and sham chimeric mice (WT-->WT). Both groups developed strong antigen-specific immune responses (antibody and IFN-gamma production). However, WT-->CD40-/- chimeras demonstrated reduced renal monocyte chemotactic protein 1 and IFN-inducible protein 10 mRNA levels and minimal T cell and macrophage influx and were protected from renal injury. Sham chimeric mice developed reduced GFR, with prominent renal expression of monocyte chemotactic protein 1 and IFN-inducible protein 10 mRNA and effector cell accumulation. In conclusion, the expression of CD40 by nonimmune renal cells plays a major role in Th1 effector responses by inducing Th1 chemokine production. Therefore, CD40-CD154 interactions are a potential therapeutic target in GN.

Human dendritic cells respond to Porphyromonas gingivalis LPS by promoting a Th2 effector response in vitro.

Understanding how mucosal pathogens modulate the immune response may facilitate the development of vaccines for disparate human diseases. In the present study, human monocyte-derived DC (MDDC)were pulsed with LPS of the oral pathogen Porphyromonas gingivalis and Escherichia coli 25922 and analyzed for: (i) production of Th-biasing/inflammatory cytokines; (ii) maturation/costimulatory molecules; and (iii) induction of allogeneic CD4+ and naive CD45RA+ T cell proliferation and release of Th1 or Th2 cytokines. We show that E. coli LPS-pulsed MDDC released Th1-biasing cytokines - consisting of high levels of IL-12 p70, IFN-gamma-inducible protein 10 (IP-10) - but also TNF-alpha, IL-10, IL-6 and IL-1beta. In contrast, no IL-12 p70 or IP-10, and lower levels of TNF-alpha and IL-10 were induced by P. gingivalis LPS. These differences were sustained at LPS doses that yielded nearly equivalent maturation of MDDC; moreover the T cell response was consistent: E. coli LPS-pulsed MDDC induced higher T cell proliferation, and T cells released more IFN-gamma and IL-2, but less IL-5 than T cells co-cultured with P. gingivalis LPS pulsed-MDDC. IL-13 was secreted by naive CD45RA+CD45RO-CD4+ T cells in response to P. gingivalisLPS-pulsed MDDC. These results suggest that human MDDC can be polarized by LPS from the mucosal pathogen P. gingivalis to induce a Th2 effector response in vitro.

Preferential Th1 immune response in invariant chain-deficient mice.

MHC class II molecules associate with the invariant chain (Ii) molecule during biosynthesis. Ii facilitates the folding of class II molecules, interferes with their peptide association, and is involved in MHC class II transport. In this study, we have investigated the in vitro and in vivo immune response of Ii-deficient mice (Ii(-/-)). Our results have demonstrated that CD4(+) T cells from Ii(-/-) mice proliferate normally in vitro after in vivo immunization with protein Ags. However, cytokine secretion profiles of Ag-primed CD4(+) T cells from Ii(-/-) mice differ from CD4(+) T cells from wild-type mice. Whereas cells from wild-type mice secrete IFN-gamma and IL-4, cells from Ii(-/-) mice secrete mostly IFN-gamma. Moreover, Ii(-/-) mice exhibit a normal Th1 response in the delayed-type hypersensitivity and trinitrobenzene sulfonic acid colitis models; however, these mice lack an in vivo Th2 response, as demonstrated in the asthma model. Therefore, we suggest that defective Ag presentation in Ii(-/-) mice leads selectively to a Th1 effector response.

TH1 AND TH2 RESPONSES IN PATHOGENESIS AND REGULATION OF EXPERIMENTAL AUTOIMMUNE UVEORETINITIS

Experimental autoimmune uveoretinitis (EAU) in animals can be induced by immunization with retinal antigens or their fragments and represents human uveitis of putative autoimmune origin. The pathogenesis of EAU, and likely also of human uveitis, involves cell-mediated destruction of retinal tissues that is dependent on retinal antigen-specific T cells. Because in most cases a Th1-type response has been implicated in path ogenesis, the prevailing consensus has been that immunoregulatory manipulations designed to enhance the Th2 response at the expense of the Th1 response will be bene-ficial in clinical treatment of uveitis. This assumption may not always be correct. The present review will summarize the evidence that, despite a central role for Th1 response in uveitis, an unopposed Th2-like response can be equally or more destructive to the retinal tissues. Furthermore, the Th1 response itself triggers regulatory circuits that feed back and dampen further recruitment of antigen-specific T cells into the Th1 effector pool. Thus, although the Th1 effector response can and does result in retinal pathology, immunoregulatory strategies must take into account that immune deviation therapies designed to replace the Th1 with a Th2 response might result in exchanging one type of pathology for another rather than in achieving the desired therapeutic effect.

Anti-interleukin 10 receptor monoclonal antibody is an adjuvant for T helper cell type 1 responses to soluble antigen only in the presence of lipopolysaccharide.

Soluble foreign antigen usually leads to a transient clonal expansion of antigen-specific T cells followed by the deletion and/or functional inactivation of the cells. As interleukin (IL)-10 is a key immunoregulatory cytokine, we questioned whether neutralization of IL-10 during priming with soluble antigen could prime for a subsequent T helper cell type 1 (Th1) effector recall response. By using an adoptive transfer model to track the fate of antigen-specific T cell receptor (TCR)-transgenic CD4(+) T cells, we show that administration of soluble ovalbumin (OVA) protein, but not OVA(323-339) peptide antigen, together with an anti-IL-10 receptor (R) mAb led to the enhancement of a Th1 response upon rechallenge. Lipopolysaccharide (LPS) present in the protein was necessary for priming for Th1 recall responses in the presence of anti-IL-10R mAb, as removal of LPS abrogated this effect. Moreover, addition of LPS to the peptide did not itself allow priming for recall Th1 effector responses unless endogenous levels of IL-10 were neutralized with an anti-IL-10R mAb. A significant increase in OVA-specific IgG1 and IgG2a isotypes was observed when the protein antigen was administered with anti-IL-10R mAb; however, this was not the case with peptide antigen administered together with anti-IL-10R and LPS. Our data, showing that LPS receptor signaling and neutralization of endogenous immunosuppressive cytokines is essential for Th1 priming, has important implications for the design of relevant vaccines for effective in vivo immunotherapy.

Suppressed T helper 2 immunity and prolonged survival of a nematode parasite in protein-malnourished mice.

Protein malnutrition may increase susceptibility to gastrointestinal parasitic infections, possibly as a result of impaired intestinal and/or systemic T helper 2 (Th2) effector responses induced by down-regulation of Th2 cytokines and/or up-regulation of Th1 cytokines. To test this hypothesis, female BALB/c mice (n = 18/diet) were fed a control (24%), marginal (7%), or deficient (3%) protein diet and given a challenge infection with Heligmosomoides polygyrus. The 3% mice had higher worm burdens at 1, 2, and 4 weeks postchallenge infection (pci), lower increases in serum IgE, reduced intestinal eosinophilia, and depressed mucosal mast cell proliferation and activation at 1-2 weeks pci. To determine whether these suppressed effector responses resulted from altered spleen and mesenteric lymph node (MLN) cytokine production, cells were restimulated in vitro with parasite antigen and cytokine concentrations were measured. Deficient MLN cells secreted significantly less IL-4 and more IFN-gamma at 1-2 weeks pci than did control MLN cells. Deficient spleen cells also secreted more IFN-gamma at 2 weeks pci compared with control spleen cells. From reverse transcription-PCR analyses, the 3% mice also had lower IL-4 mRNA level in spleen and MLN at 1-2 weeks pci. Our study supports the hypothesis that protein malnutrition increases the survival of a nematode parasite by decreasing gut-associated IL-4 (Th2) and increasing IFN-gamma (Th1) within 2 weeks pci, leading to reduced intestinal and systemic Th2 effector responses.

An appraisal of T cell subsets and the potential for autoimmune injury

The initiation of T cell dependent immune responses requires the T cell to utilize a heterodimeric cell surface antigen receptor to recognize an antigenic peptide in association with major histocompatibility complex (MHC) molecules on the surface of antigen presenting cells (APCs). This is a necessary but insufficient signal for T cell activation. T cells must additionally receive a costimulatory signal that can be delivered through a variety of receptor-ligand pairs. Of these, the best characterized is the CD28-B7 (CD80 and CD86) couple. It has been appreciated for decades that the outcome of the T cell-APC interaction is highly variable and can lead to different forms of immune responses. The past decade has witnessed major advances in the definition of how distinct lymphocyte functions are dictated by expression of distinct cytokines, activation of defined signal transduction pathways, and, most recently, expression of distinct transcription factors. The production of ever increasing numbers of induced mutant mouse strains has created new reagents in which to analyze the role of particular cytokines or other proteins in defined immune responses. While the temptation remains to accomodate emerging information into reductionist models of T cell dependent immunopathology, it is our view that in vivo immune responses are highly complex and regulated events that defy simple categorization.

IGIF Does Not Drive Th1 Development but Synergizes with IL-12 for Interferon-γ Production and Activates IRAK and NFκB

In these studies, IFNγ-inducing factor (IGIF), unlike IL-12, did not drive Th1 development in BALB/c or C57BL/6 mice, but like IL-1α, potentiated IL-12–driven Th1 development in BALB/c mice. IGIF and IL-12 synergized for IFNγ production from Th1 cells. Unlike IL-1α, IGIF had no effect on Th2 cells. IGIF signaled through IRAK, IL-1 receptor–associated kinase, to induce nuclear translocation of p65/p50 NFκB in Th1 cells. IL-1α had no effect on proliferation, cytokine production, or NFκB activation in Th1 cells but activated NFκB and proliferation in Th2 cells. Thus, Th1 and Th2 cells may differ in responsiveness and receptor expression for IL-1 family molecules. IGIF and IL-1α may differentially amplify Th1 and Th2 effector responses, respectively.

Characterization of T cell-mediated responses in nonhealing and healing Leishmania major infections in the absence of endogenous IL-4.

IL-4 drives polarized Th2 responses, and differentiating Th2 cells down-regulate their sensitivity to IL-12. Therefore, the failure of BALB/c mice to heal Leishmania major infection could be due to an IL-4-dependent biased Th2 response or to a reduced capacity of Leishmania-specific Th cells to respond to IL-12. We examined the ability of CD4+ Th cells from L. major-infected wild-type and IL-4-deficient BALB/c mice to respond to IL-12. We show that the inability of normal and IL-4-deficient BALB/c mice to heal L. major infections is due to their inability to generate effective Th1 responses and not to persistent IL-4-dominated Th2 responses. Redirection of immune responses in vivo by administration of IL-12 or anti-CD4 mAb treatment in the early phase of infection (+/-12 days) allows both normal and IL-4-deficient BALB/c mice to heal their lesions by allowing them to develop an efficient Th1 response regardless of the presence or the absence of IL-4. Finally, on a population level, Ag-specific Th cells from infected animals induced to heal display a strongly elevated response to IL-12.

Differential effect of CTLA4Ig on murine graft-versus-host disease (GVHD) development: CTLA4Ig prevents both acute and chronic GVHD development but reverses only chronic GVHD.

The role of costimulation was examined in an in vivo model of alloantigen-driven Th1 or Th2 cytokine responses, the parent-into-F1 model of acute or chronic graft-vs-host disease (GVHD), respectively. The soluble fusion protein, murine CTLA4Ig, which blocks engagement of CD28 by its natural ligand B7-1 and B7-2, was administered either early, at the time of GVHD induction, or delayed, after the establishment of Th1 or Th2 effector responses (day 7). Early administration of CTLA4Ig prevented the development of both acute and chronic GVHD by preventing the activation of donor T cells, i.e., by blocking characteristic Th1 or Th2 cytokine production and blocking memory marker up-regulation on donor T cells. Delayed CTLA4Ig administration was unable to alter acute GVHD but did reverse chronic GVHD as evidenced by normalization of serum autoantibody levels, normal host B cell numbers and MHC class II expression, reduced donor T cell expression of CD40 ligand, and reduced numbers of donor CD4+ memory T cells. The percentage of donor memory cells was not altered by delayed CTLA4Ig. We conclude that in this model, alloantigen-driven Th1 or Th2 responses are equally susceptible to costimulatory blockade at the onset of disease; however, once effector mechanisms become established, only Th2-driven responses have a requirement for further costimulation for the continued expansion of CD4+ T cells. These data suggest that humoral, lupus-like autoimmunity requires continuous T cell help for B cells, and agents that interrupt this process may be beneficial.

Gene therapy: Advances and limitations

Recombinant adenovirus vectors are efficient at transferring genes into somatic tissues but are limited for use in clinical gene therapy by immunologic factors that result in the rapid loss of gene expression and inhibit secondary gene transfer. This study demonstrates that systemic coadministration of recombinant adenovirus with soluble CTLA4lg, which is known to block co-stimulatory signals between T cells and antigen presenting cells, leads to persistent adenovirus gene expression in mice without long-term immunosuppression. This form of immunotherapy greatly enhances the likelihood that recombinant adenovirus vectors will be useful for human gene therapy. Recombinant adenoviruses are attractive vehicles for liver-directed gene therapy because of the high efficiency with which they transfer genes to hepatocytes in vivo. First-generation recombinant adenoviruses deleted of E1 sequences also express recombinant and early and late viral genes, which lead to development of destructive cellular immune responses. Previous studies indicated that class I major histocompatibility complex (MHC)-restricted cytotoxic T lymphocytes (CTLs) play a major role in eliminating virusinfected cells. The present studies utilize mouse models to evaluate the role of T-helper cells in the primary response to adenovirus-mediated gene transfer to the liver. In vivo ablation of CD4+ cells or interferon γ (INF-γ) was sufficient to prevent the elimination of adenovirus-transduced hepatocytes, despite the induction of a measurable CTL response. Mobilization of an effective TH1 response as measured by in vitro proliferation assays was associated with substantial upregulation of MHC class I expression, an effect that was prevented in IFN-γ-deficient animals. These results suggest that elimination of virus-infected hepatocytes in a primary exposure to recombinant adenovirus requires both induction of antigen-specific CTLs as well as sensitization of the target cell by the TH1-mediated activation of MHC class I expression.