Infection Of Antigen-presenting Cells Research Articles

Cytomegalovirus restricts ICOSL expression on antigen-presenting cells disabling T cell co-stimulation and contributing to immune evasion.

Viral infections are controlled, and very often cleared, by activated T lymphocytes. The inducible co-stimulator (ICOS) mediates its functions by binding to its ligand ICOSL, enhancing T-cell activation and optimal germinal center (GC) formation. Here, we show that ICOSL is heavily downmodulated during infection of antigen-presenting cells by different herpesviruses. We found that, in murine cytomegalovirus (MCMV), the immunoevasin m138/fcr-1 physically interacts with ICOSL, impeding its maturation and promoting its lysosomal degradation. This viral protein counteracts T-cell responses, in an ICOS-dependent manner, and limits virus control during the acute MCMV infection. Additionally, we report that blockade of ICOSL in MCMV-infected mice critically regulates the production of MCMV-specific antibodies due to a reduction of T follicular helper and GC B cells. Altogether, these findings reveal a novel mechanism evolved by MCMV to counteract adaptive immune surveillance, and demonstrates a role of the ICOS:ICOSL axis in the host defense against herpesviruses.

Bacillus Calmette-Gu\xe9rin Induces PD-L1 Expression on Antigen-Presenting Cells via Autocrine and Paracrine Interleukin-STAT3 Circuits

Bacillus Calmette-Guérin (BCG) is the only licensed vaccine for tuberculosis (TB), and is also used as an immunotherapy for bladder cancer and other malignancies due to its immunostimulatory properties. Mycobacteria spp., however, are well known for their numerous immune evasion mechanisms that limit the true potential of their therapeutic use. One such major mechanism is the induction of programmed death ligand-1 (PD-L1), which mitigates adaptive immune responses. Here, we sought to unravel the molecular pathways behind PD-L1 up-regulation on antigen-presenting cells (APCs) by BCG. We found that infection of APCs with BCG induced PD-L1 up-regulation, but that this did not depend on direct infection, suggesting a soluble mediator for this effect. BCG induced potent quantities of IL-6 and IL-10, and the downstream transcription factor STAT3 was hyper-phosphorylated. Intracellular analyses revealed that levels of PD-L1 molecules were associated with the STAT3 phosphorylation state, suggesting a causal link. Neutralisation of the IL-6 or IL-10 cytokine receptors dampened STAT3 phosphorylation and BCG-mediated up-regulation of PD-L1 on APCs. Pharmacological inhibition of STAT3 achieved the same effect, confirming an autocrine-paracrine cytokine loop as a mechanism for BCG-mediated up-regulation of PD-L1. Finally, an in vivo immunisation model showed that BCG vaccination under PD-L1 blockade could enhance antigen-specific memory CD4 T-cell responses. These novel findings could lead to refinement of BCG as both a vaccine for infectious disease and as a cancer immunotherapy.

Ectromelia-encoded protein B22 restricts CD4+ T cell activation.

Abstract Orthopoxviruses encode many immunomodulatory proteins that profoundly interfere with various aspects of the immune system. Ectromelia (ECTV) is the orthopoxvirus that naturally infects mice, affording the opportunity to study viral-host interactions that have co-evolved. Importantly, disease progression closely mimics that of monkeypox and smallpox in humans. CD4+ T cells are one immune cell type crucial for the control of ECTV and have important roles in both direct cytolytic killing of infected cells and helping B and CD8+ T cell activation. Here we show that ECTV infection of antigen presenting cells inhibits their ability to activate CD4+ T cells directly. However, the inhibitory mechanism is not straightforward as neither down-regulation of surface MHC-II nor soluble factors are responsible. A promising candidate protein for this inhibition is B22, which is located on the plasma membrane and has homologs in most virulent strains including smallpox and monkeypox, but not vaccine strains. Indeed, we have determined that B22 contributes substantially to ECTV virulence. Here we show that B22 is both necessary and sufficient to restrict CD4+ T cell activation, with experiments ongoing to elucidate the specific mechanism of inhibition.

Abstract A157: Oncolytic poliovirus mediated immune events

Abstract Introduction: Tumor-targeted therapies that efficiently eliminate malignant cells and in the process engage the innate and adaptive immune system are desirable for preventing cancer recurrence. We have pioneered an oncolytic poliovirus therapy, PVSRIPO, which selectively targets and eliminates malignant cells. PVSRIPO is a recombinant polio:rhinovirus chimera, engineered to eliminate neuronal competence and is devoid of neuropathogenicity after intracerebral inoculation in non-human primates and in humans. PVSRIPO has cancer tropism due to widespread ectopic expression of the poliovirus receptor, CD155, in solid cancers. PVSRIPO kills tumor cells with no effect on normal cells. Unlike other oncolytic viruses, PVSRIPO is effective in the presence of neutralizing antibodies and an innate antiviral response. A first-in-human Phase-1 study with PVSRIPO at Duke University has shown remarkable promise in patients with recurrent glioblastoma (GBM), a uniformly lethal disease. PVSRIPO tumor cell killing is associated with the induction of danger- and pathogen-associated molecular patterns (DAMPs and PAMPs) via antiviral type I interferons (IFNs) and simultaneous non-lethal infection of antigen-presenting cells (APCs) such as monocytes and dendritic cells (DCs) via CD155. Type I IFNs are known to inhibit tumor proliferation, activate innate immune cells and bridge innate and adaptive immunity among other functions. Methods: To understand immune events associated with poliovirus infection of APCs, we examined the effects of PVSRIPO treatment on the human macrophage cell line (THP1) and primary human monocyte-derived DCs. PVSRIPO-treated DCs were evaluated for expression of maturation/activation markers and compared to untreated immature and mature DCs. To investigate if DCs exposed to PVSRIPO-induced dying tumor cells are capable of presenting tumor antigens to T cells, we performed an in vitro human immunotherapy assay. Human DCs generated from HLA-A2+ donor cells were incubated with PVSRIPO-induced tumor cell lysate and then used to stimulate autologous T cells in vitro followed by a cytotoxic T lymphocyte (CTL) assay. All tumor cell lines used for the human immunotherapy experiments were positive for HLA-A2 expression. The following tumor cell lines were used for this analysis: DM6 (MART+) melanoma cell line, MDA-MB231 (CEA+) triple-negative breast cancer cell line, SUM149 (EGFR+) inflammatory breast cancer cell line and LNCaP (PSA+) prostate cancer cell line. Results: PVSRIPO infection of THP1 macrophages induces Mda5/Stat1 and Stat1 phosphorylation as well as TNFα and IFNβ release. Coculturing of DCs with PVSRIPO-induced tumor cell lysate stimulates DC activation and IL12 production. Human DCs loaded with PVSRIPO-induced tumor cell lysate are capable of stimulating tumor antigen-specific cytotoxic T cells in vitro. Autologous DCs transfected with RNA that encodes for CEA, EGFR, MART or PSA (tumor antigen-expressing cells) and human tumor cell lines were used as targets to assess tumor antigen-specificity of T cells. Conclusion: Our data suggests that along with destruction of the primary tumor, oncolytic poliovirus mediates immune events. Importantly, we demonstrate that human DCs co-incubated with PVSRIPO-induced tumor cell lysate stimulate tumor antigen-specific cytotoxic T cells in an in vitro human immunotherapy assay. In ongoing studies we are analyzing oncolytic poliovirus mediated immune events in syngeneic, immunocompetent murine models of breast cancer and prostate cancer. Citation Format: Michael C. Brown, Eda K. Holl, David Boczkowski, Darell D. Bigner, Matthias Gromeier, Smita K. Nair. Oncolytic poliovirus mediated immune events. [abstract]. In: Proceedings of the CRI-CIMT-EATI-AACR Inaugural International Cancer Immunotherapy Conference: Translating Science into Survival; September 16-19, 2015; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(1 Suppl):Abstract nr A157.

Short Communication: Inhibition of DC-SIGN-Mediated HIV-1 Infection by Complementary Actions of Dendritic Cell Receptor Antagonists and Env-Targeting Virus Inactivators.

The DC-SIGN receptor on human dendritic cells interacts with HIV gp120 to promote both infection of antigen-presenting cells and transinfection of T cells. We hypothesized that in DC-SIGN-expressing cells, both DC-SIGN ligands such as dextrans and gp120 antagonists such as peptide triazoles would inhibit HIV infection with potential complementary antagonist effects. To test this hypothesis, we evaluated the effects of dextran (D66), isomaltooligosaccharides (D06), and several peptide triazoles (HNG156, K13, and UM15) on HIV infection of B-THP-1/DC-SIGN cells. In surface plasmon resonance competition assays, D66 (IC50 = 35.4 μM) and D06 (IC50 = 3.4 mM) prevented binding of soluble DC-SIGN to immobilized mannosylated bovine serum albumin (BSA). An efficacious dose-dependent inhibition of DC-SIGN-mediated HIV infection in both pretreatment and posttreatment settings was observed, as indicated by inhibitory potentials (EC50) [D66 (8 μM), D06 (48 mM), HNG156 (40 μM), UM15 (100 nM), and K13 (25 nM)]. Importantly, both dextrans and peptide triazoles significantly decreased HIV gag RNA levels [D66 (7-fold), D06 (13-fold), HNG156 (7-fold), K-13 (3-fold), and UM15 (6-fold)]. Interestingly, D06 at the highest effective concentration showed a 14-fold decrease of infection, while its combination with 50 μM HNG156 showed a 26-fold decrease. Hence, these compounds can combine to inactivate the viruses and suppress DC-SIGN-mediated virus-cell interaction that as shown earlier leads to dendritic cell HIV infection and transinfection dependent on the DC-SIGN receptor.

Abstract 2883: Improved generation of central memory CD8+ T cells with CD40L expressing recombinant vaccinia virus

Abstract Immunological memory is the ability of the immune system, upon re-encounter with the same antigen, to respond with greater speed and intensity and constitutes the basis for vaccination. Regarding CD8+ T-cells, the memory compartment has been divided in different subsets of memory cells: Central Memory CD8+ T cells (CD45RO+CD62L+; Tcm) and Effector Memory CD8+ T cells (CD45RO+CD62L- ; Tem). In cancer immunotherapy, studies performed on animal models showed that Tcm tumor-reactive CD8+ T cells, as compared to effector memory T cells, confer superior antitumor immunity leading to better outcome.. In the last decades has been clarified that CD4+T cells help, during CD8+ T cells memory differentiation, is mainly mediated by the CD40L expression on their surface upon activation and consecutive triggering of CD40 expressed by antigen presenting cells and activated CD8+ T cells. Recombinant vaccinia virus (rVV) is one of the most immunogenic viral vectors enabling specific targeting and modulation of anti-tumor immune responses. We previously demonstrated that infection of antigen presenting cells (APCs) , with a non-replicating rVV encoding human CD40L (rVV-CD40L) was leading to an efficient licensing of APC, as demonstrated by increased expression of surface ligand/receptors (MHC I/II, CD80 and CD86),Th1 and homeostatic cytokines (IL-12, TNFa, IL-7 and IL-15). Using sorted naive CD8+ Tcells (CD45RA+CD62L+) primed with infected allogeneic CD14+ we observed that activated cells had an increased central memory phenotype (CD45RO+ CD62L+) with rVV40L infection as compared to culture primed with wild type virus infected CD14+ cells or treated with soluble 40 Ligand recombinant protein (s40L). The majority of Tcm express even the a-chain of IL-7 receptor (IL-7Ra) which has been reported to be express on “Bona Fide” memory cells and is associated with an increased cells survival. In an autologous system targeting a tumor antigen response, we demonstrated how CD14+rVV40L, as compared controls or soluble 40L stimulation, strongly promote the expansion of MelanA/Mart-1 specific CD8+ characterized with a Tcm phenotype and expression of IL-7Ra. Functional characterization of these tumor specific CD8+ demonstrated higher levels of IFN-g and IL-2. Studies performed in murine models have shown that, upon activation, CD8+ T cells can also express the CD40 receptor and therefore become directly sensitive to CD40L activation. However,in our hands, induction and characterization of CD40 receptor, on human CD8+ T cells remained negative. Overall, these data are confirming the potent anti-tumor capacities of recombinant vaccinia viruses expressing CD40L notably via CD40 driven APC licensing which promotes an effective generation of antigen specific CD8+ T cells with a memory phenotype. These in-vitro observations validate the strong clinical potential of our recombinant vaccinia virus constructs co-expressing CD40L for cancer immunotherapies. Citation Format: Emanuele Trella, Evangelos Panoupolos, Swantje Heidtmann, Nermin Raafat, Giulio Cesare Spagnoli, Paul Zajac. Improved generation of central memory CD8+ T cells with CD40L expressing recombinant vaccinia virus. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2883. doi:10.1158/1538-7445.AM2014-2883

Pyridinyl imidazole inhibitors of p38 MAP kinase impair viral entry and reduce cytokine induction by Zaire ebolavirus in human dendritic cells

Antigen presenting cells (APCs), including macrophages and dendritic cells, are early and sustained targets of Ebola virus (EBOV) infection in vivo. Because EBOV activates mitogen-activated protein kinase (MAPK) signaling upon infection of APCs, we evaluated the effect of pyridinyl imidazole inhibitors of p38 MAPK on EBOV infection of human APCs and EBOV mediated cytokine production from human DCs. The p38 MAPK inhibitors reduced viral replication in PMA-differentiated macrophage-like human THP-1 cells with an IC50 of 4.73μM (SB202190), 8.26μM (p38kinhIII) and 8.21μM (SB203580) and primary human monocyte-derived dendritic cells (MDDCs) with an IC50 of 2.67μM (SB202190). Furthermore, cytokine production from EBOV-treated MDDCs was inhibited in a dose-dependent manner. A control pyridinyl imidazole compound failed to inhibit either EBOV infection or cytokine induction. Using an established EBOV virus-like particle (VLP) entry assay, we demonstrate that inhibitor pretreatment blocked VLP entry suggesting that the inhibitors blocked infection and replication at least in part by blocking EBOV entry. Taken together, our results indicate that pyridinyl imidazole p38 MAPK inhibitors may serve as leads for the development of therapeutics to treat EBOV infection.

Pulmonary M. tuberculosis infection delays Th1 immunity via immunoadaptor DAP12-regulated IRAK-M and IL-10 expression in antigen-presenting cells.

Interaction of mycobacteria with the host leads to retarded expression of T helper cell type 1 (Th1) immunity in the lung. However, the immune mechanisms remain poorly understood. Using in vivo and in vitro models of Mycobacterium tuberculosis (M. tb) infection, we find the immunoadaptor DAP12 (DNAX-activating protein of 12 kDa) in antigen-presenting cells (APCs) to be critically involved in this process. Upon infection of APCs, DAP12 is required for IRAK-M (interleukin-1 receptor-associated kinase M) expression, which in turn induces interleukin-10 (IL-10) and an immune-suppressed phenotype of APCs, thus leading to suppressed Th1 cell activation. Lack of DAP12 reduces APC IL-10 production and increases their Th1 cell-activating capability, resulting in expedited Th1 responses and enhanced protection. On the other hand, adoptively transferred DAP12-competent APCs suppress Th1 cell activation within DAP12-deficient hosts, and blockade of IL-10 aborts the ability of DAP12-competent APCs to suppress Th1 activation. Our study identifies the DAP12/IRAK-M/IL-10 to be a novel molecular pathway in APCs exploited by mycobacterial pathogens, allowing infection a foothold in the lung.

Herpes Simplex Virus 1 Glycoprotein B and US3 Collaborate To Inhibit CD1d Antigen Presentation and NKT Cell Function

Herpes simplex viruses (HSVs) are prevalent human pathogens that establish latency in human neuronal cells and efficiently evade the immune system. It has been a major medical challenge to eradicate them and, despite intensive efforts, an effective vaccine is not available. We previously showed that upon infection of antigen-presenting cells, HSV type 1 (HSV-1) rapidly and efficiently downregulates the major histocompatibility complex class I-like antigen-presenting molecule, CD1d, and potently inhibits its recognition by CD1d-restricted natural killer T (NKT) cells. It suppresses CD1d expression primarily by inhibiting its recycling to the cell surface after endocytosis. We identify here the viral glycoprotein B (gB) as the predominant CD1d-interacting protein. gB initiates the interaction with CD1d in the endoplasmic reticulum and stably associates with it throughout CD1d trafficking. However, an additional HSV-1 component, the serine-threonine kinase US3, is required for optimal CD1d downregulation. US3 expression in infected cells leads to gB enrichment in the trans-Golgi network (TGN) and enhances the relocalization of both gB and CD1d to this compartment, suggesting that following internalization CD1d is translocated from the endocytic pathway to the TGN by its association with gB. Importantly, both US3 and gB are required for efficient inhibition of CD1d antigen presentation and NKT cell activation. In summary, our results suggest that HSV-1 uses gB and US3 to rapidly inhibit NKT cell function in the initial antiviral response.

Seoul virus suppresses NF-κB-mediated inflammatory responses of antigen presenting cells from Norway rats

Hantavirus infection reduces antiviral defenses, increases regulatory responses, and causes persistent infection in rodent hosts. To address whether hantaviruses alter the maturation and functional activity of antigen presenting cells (APCs), rat bone marrow-derived dendritic cells (BMDCs) and macrophages (BMDMs) were generated and infected with Seoul virus (SEOV) or stimulated with TLR ligands. SEOV infected both DCs and macrophages, but copies of viral RNA, viral antigen, and infectious virus titers were higher in macrophages. The expression of MHCII and CD80, production of IL-6, IL-10, and TNF-alpha, and expression of Ifnbeta were attenuated in SEOV-infected APCs. Stimulation of APCs with poly I:C prior to SEOV infection increased the expression of activation markers and production of inflammatory cytokines and suppressed SEOV replication. Infection of APCs with SEOV suppressed LPS-induced activation and innate immune responses. Hantaviruses reduce the innate immune response potential of APCs derived from a natural host, which may influence persistence of these zoonotic viruses in the environment.

Characterization of recombinant influenza A virus as a vector for HIV-1 p17 Gag

We have generated a recombinant influenza A virus with the HIV-1 p17 Gag (rFlu-p17) gene inserted into the influenza virus neuraminidase (NA) gene. Expression of HIV-1 p17 protein was detected by conventional Western blot analysis and also by liquid chromatography/tandem mass spectrometry (LC–MS/MS) analysis of rFlu-p17 infected cells. The latter method does not depend on protein-specific antibody preparations and proved to be a powerful tool to detect proteins of interest. Next, antigen presentation of p17 expressed after infection of antigen-presenting cells was determined. Cloned p17-specific CD8+ T-cells were co-cultured with rFlu-p17 infected B-cells and produced IFN-γ upon stimulation. Furthermore, we showed that immunization with rFlu-p17 elicited a humoral immune response in mice. This study shows that replication-deficient rFlu-p17 is antigenic in vitro and immunogenic in vivo and warrants further development as a candidate vaccine vector.

Cyclooxygenase-2 Inhibition Enhances Activation of T Helper Type 1 Responses During Salmonella Infection

Production of IL-12 and IFN-γ secretion are important components of the protective host response against the intracellular bacterial pathogen, Salmonella typhimurium. While infection with Salmonella does elicit this T helper type 1 response, its magnitude does not appear to be sufficient to prevent infection or limit pathogenesis. Therefore we have investigated factors which might limit a T helper type 1 response following infection. Previously we found that infection of antigen presenting cells with Salmonella dramatically increases cyclooxygenase-2 (COX-2) activity, resulting in high levels of prostaglandin E2 (PGE2). Since PGE2 production can have profound effects on initiation of T helper type 1 responses, we questioned whether this mediator might limit antigen-specific T cell activation. Here we show that blockage of COX-2 activity with the selective inhibitor celecoxib leads to enhancement of the T helper type 1 components stimulated by Salmonella infection. In vitro studies demonstrate the induction of IL-12 and IFN-γ upon Salmonella exposure, which are further increased following COX-2 inhibition. Taken together these in vitro studies suggest that COX-2 activity can limit a salmonella-initiated T helper type 1 response.

Factors influencing immune response after in vivo retrovirus-mediated gene transfer to the liver.

Highly efficient retrovirus-mediated gene transfer into hepatocytes in vivo triggers an immune response directed against transduced hepatocytes. This effect may be due either to spreading of retroviral vectors in the blood stream with subsequent infection of antigen presenting cells (APCs) or to cross-presentation of the transgene product present as a contaminant in the viral stock. In order to decrease immune response, we evaluated the effect of asanguineous perfusion of the liver as well as purification of the viral stock on long-term transduction of hepatocytes using the nls-lacZ marker gene. Animals were divided in four groups. In group 1, the viral supernatant was perfused in the regenerating liver after complete vascular exclusion of the organ. In group 2, using the same strategy, animals received retroviral supernatant that was passed through a beta-galactosidase affinity column to reduce beta-galactosidase contamination. In two control groups (respectively groups 3 and 4) the corresponding viral supernatants were delivered via peripheral injection. In group 1, 23.1% of animals had no immune response 2 months after gene delivery vs. 33.4% in group 2, 4.3% in control group 3, and 0% in control group 4. Statistical analysis of the results demonstrated that only the difference between groups 2 and 3 was statistically significant. This indicated that both asanguineous perfusion together with passage through an affinity column were required to decrease significantly immune response. Our present results suggest that both supernatant contamination and viral spreading contribute to immune response after retrovirus-mediated gene delivery to the liver.

In-vivo retroviral gene transfer to the liver is cancelled by an immune response against the corrected cells. Can it be avoided?

Highly efficient retroviral-mediated gene transfer into hepatocytes in vivo has been previously reported in rats, but some reports described transient expression of the transgene that may be related to induction of an immune response against the transgene product. To devise a surgical approach to circumvent this drawback, two-thirds partial hepatectomy was performed in Wistar male rats to induce the hepatocyte division required to achieve retrovirus integration. Delivery of amphotrophic retroviral vectors (RVV) encoding Escherichia coli beta-galactosidase was performed 24 h after partial hepatectomy. In a first group (n = 11), gene delivery was performed by peripheral injection of 2 ml retrovirus-containing medium. For the second group (n = 11), asanguineous perfusion of the regenerating liver after complete vascular exclusion was carried out with 20 ml viral solution. Liver biopsies were performed sequentially in each group. In the first group, beta-galactosidase was expressed at day 7 in 7 +/- 6.3% of hepatocytes and the labeled hepatocytes had disappeared in less than 4 weeks. Polymerase chain reaction experiments demonstrated the elimination of the transduced cells and the appearance of antibodies against beta-galactosidase. Of the 11 rats in the second group, 8 were still able to express beta-galactosidase more than 6 weeks after asanguineous perfusion with no detectable antibody response. Asanguineous perfusion of the regenerating liver with RVV after complete vascular exclusion enabled long-term expression in rats and avoided the immune response present after peripheral delivery in most animals. These results suggest that the immune reaction is secondary to viral infection of antigen presenting cells. Asanguineous perfusion could thus be a way to perform gene therapy for inherited liver diseases without immunosuppressive therapy.

Role of viruses in type I diabetes

Viral infections frequently elicit strong cellular and humoral immune responses. This bears the inherent danger of co-activating autoreactive lymphocytes, either through bystander activation by cytokines or through direct sharing of conformational determinants between self and virus (mimicry). Autoimmune diseases could then result, even after clearance of the viral infection, if enough autoreactive cells are activated. Alternatively, viral infection of antigen presenting cells can locally enhance inflammation and drive autoreactive lymphocytes. Evidence for these mechanisms, as well as emerging therapeutic concepts, will be discussed.

Enhancement of disease by neutralizing antiviral antibodies in the absence of primed antiviral cytotoxic T cells.

The effects of neutralizing antibodies on the disease course in mice infected with the noncytopathic lymphocytic choriomeningitis virus (LCMV) were evaluated. Whereas non-neutralizing antisera exhibiting high enzyme-linked immunosorbent assay titers had no effect on T cell responses and their consequences, neutralizing antisera modulated them variably. Neutralizing antibodies were able to prevent lethal choriomeningitis after intracerebral infection with a neurotropic LCMV-isolate (ARMSTRONG) although they could not control local virus replication. The same antibodies exhibited little or no protective effect on choriomeningitis induced by LCMV-WE, a viscerotrope isolate. Surprisingly, these antibodies rendered mice much more susceptible to choriomeningitis after intracerebral infection with LCMV DOCILE, a very rapidly spreading lymphocyto-viscerotrope virus; in this situation antibodies prevented overwhelming infection which causes deletion of immunopathogenic cytotoxic T cell responses. Thus preexisting neutralizing antiviral antibodies had little influence on local virus spread in peripheral tissues but they reduced hematogenic spread and infection of antigen-presenting cells; thereby they influenced the primary cytotoxic T cell (CTL) response and indirectly modulated the extent of T cell-mediated immunopathology in peripheral organs. These results may explain why vaccines inducing neutralizing antibodies but no CTL may enhance an immunopathological disease caused by challenge infection with a noncytopathic virus.

Hemolysin-producing Listeria monocytogenes affects the immune response to T-cell-dependent and T-cell-independent antigens.

A murine experimental infection with a hemolysin-producing (Hly+) strain of Listeria monocytogenes and a non-hemolysin-producing (Hly-) mutant was used as an in vivo model to evaluate the role of hemolysin production in the immune response. No antilisterial antibodies were detectable following sublethal infection with Hly+ bacteria, but consistent antilisterial immunoglobulin G (IgG) and IgM antibody production was observed following sublethal infection with the Hly- mutant. Hly+ but not Hly- L. monocytogenes induced transient inhibition of antibody response to Hly- bacteria and to unrelated T-cell-dependent (tetanus toxoid) and T-cell-independent (pneumococcal polysaccharide 3) antigens. Transient inhibition of the activation of an antigen-specific T-cell clone was also observed following Hly+ infection of antigen-presenting cells but not following Hly- infection. These results suggest that hemolysin production by L. monocytogenes is an important factor in modulating the immune response to T-cell-dependent and T-cell-independent antigens in infected individuals.

Transmission of HIV by antigen presenting cells during T-cell activation: prevention by 3'-azido-3'-deoxythymidine.

Tetanus toxoid (TT) reactive CD4+ cells were infected with HTLV-IIIB and exposed to TT at various times throughout a 7-day interval. Acute infection per se failed to produce overt cytopathology. However, exposure of infected cells to TT resulted in a rapid loss of cell viability, an increase in viral p24 expression, and a decline in T-cell blastogenesis. To determine whether HIV infection of antigen presenting cells (APC) could impact on T-cell activation, virus infected APC were utilized to present TT to responsive CD4+ cells. Use of infected APC produced effects similar to antigen stimulation of infected T-cells. These results suggest that the conditions of antigen presentation during T-cell activation may provide an excellent opportunity for virus transmission which may produce maximal immune dysfunction. However, preincubating antigen specific T-cells with the virostatic agent 3'-azido-3'-deoxythymidine (AZT) could prevent most of these effects.