Antigen-dependent Stimulation Research Articles

Impact of variants of concern on SARS-CoV-2 viral dynamics in non-human primates.

The impact of variants of concern (VoC) on SARS-CoV-2 viral dynamics remains poorly understood and essentially relies on observational studies subject to various sorts of biases. In contrast, experimental models of infection constitute a powerful model to perform controlled comparisons of the viral dynamics observed with VoC and better quantify how VoC escape from the immune response. Here we used molecular and infectious viral load of 78 cynomolgus macaques to characterize in detail the effects of VoC on viral dynamics. We first developed a mathematical model that recapitulate the observed dynamics, and we found that the best model describing the data assumed a rapid antigen-dependent stimulation of the immune response leading to a rapid reduction of viral infectivity. When compared with the historical variant, all VoC except beta were associated with an escape from this immune response, and this effect was particularly sensitive for delta and omicron variant (p<10-6 for both). Interestingly, delta variant was associated with a 1.8-fold increased viral production rate (p = 0.046), while conversely omicron variant was associated with a 14-fold reduction in viral production rate (p<10-6). During a natural infection, our models predict that delta variant is associated with a higher peak viral RNA than omicron variant (7.6 log10 copies/mL 95% CI 6.8-8 for delta; 5.6 log10 copies/mL 95% CI 4.8-6.3 for omicron) while having similar peak infectious titers (3.7 log10 PFU/mL 95% CI 2.4-4.6 for delta; 2.8 log10 PFU/mL 95% CI 1.9-3.8 for omicron). These results provide a detailed picture of the effects of VoC on total and infectious viral load and may help understand some differences observed in the patterns of viral transmission of these viruses.

A lymph node slice culture model to characterize T cell activation dynamics and anti-viral responses in human tissue

Abstract T cell activation in the lymph node (LN) is critical to initiating adaptive immune responses to vaccination, tumors, and infection. We previously demonstrated that T cells from human tissue sites (blood, LNs, bone marrow, lung) undergo conserved cell state transitions during activation. However, these studies were performed in suspension culture from dissociated tissue, depleting the spatial and signaling contributions of the tissue microenvironment. To address this, we developed techniques for culture and single-cell analysis of human LN slices, which preserve tissue architecture, local signaling, and physiological cell frequencies. Single-cell RNA-seq (scRNA-seq) of matched LN suspension and slice cultures stimulated with anti-CD3/28 antibody tetramers revealed distinct activation patterns. While anti-CD3/28 stimulation is commonly used to study T cell activation, this reagent circumvents antigen presentation and may not fully recapitulate antigen-mediated signaling. Conversely, antigen-dependent stimulation with superantigen mimics require MHCII/TCR binding and may benefit from the intact microenvironment in slice culture. We discovered a distinct metabolic intermediate through scRNA-seq analysis of superantigen-stimulated LN suspension and slice cultures that is not observed with anti-CD3/28 stimulation. Overall, these studies demonstrate the utility of analyzing T cell responses in intact tissue and comparative analysis of distinct mechanisms of activation. 1T32GM145440-01 (MSTP Training Grant) U19AI128949 (NIAID)

CD27+TIM-1+ memory B cells promoted the development of Foxp3+ Tregs and were associated with better survival in acute respiratory distress syndrome.

Acute respiratory distress syndrome (ARDS) is a rapid onset life-threatening condition involving uncontrolled propagation of inflammatory responses. Here, we observed that ARDS patients that survived presented significantly higher frequencies of TIM-1+ B cells, especially the CD27+TIM-1+ B cells, than the ARDS patients who succumbed to the condition. We then found that using BCR/CD40 antigen-dependent stimulation or Staphylococcus aureus Cowan (SAC) antigen-independent stimulation, TIM-1+ B cells presented significantly higher IL-10 secretion and/or TGF-β1 secretion, with SAC stimulation being more effective. CD4+ T cells that incubated with TIM-1+ B cells presented significantly elevated IL-10 secretion, TGF-β1 secretion, and Foxp3 expression, than CD4+ T cells that incubated with TIM-1- B cells, suggesting TIM-1+ B cells promoted the in vitro development of Foxp3+ Treg cells. Interestingly, this TIM-1+ B cell-mediated promotion of Foxp3 expression was mostly dependent on TGF-β1 but not IL-10, since neutralization of TGF-β1, but not IL-10, resulted in the suppression of Foxp3 expression. We further showed that in TIM-1+ B cells, the CD27+ classical memory B cell subset demonstrated more regulatory potency than the CD27- subset. Together, our results suggested that the TIM-1+ B cells, especially those that expressed CD27, could promote Foxp3 expression. Their clinical efficacy in treating ARDS should be examined in in vivo experiments.

HIV-1 Activates T Cell Signaling Independently of Antigen to Drive Viral Spread

SummaryHIV-1 spreads between CD4 T cells most efficiently through virus-induced cell-cell contacts. To test whether this process potentiates viral spread by activating signaling pathways, we developed an approach to analyze the phosphoproteome in infected and uninfected mixed-population T cells using differential metabolic labeling and mass spectrometry. We discovered HIV-1-induced activation of signaling networks during viral spread encompassing over 200 cellular proteins. Strikingly, pathways downstream of the T cell receptor were the most significantly activated, despite the absence of canonical antigen-dependent stimulation. The importance of this pathway was demonstrated by the depletion of proteins, and we show that HIV-1 Env-mediated cell-cell contact, the T cell receptor, and the Src kinase Lck were essential for signaling-dependent enhancement of viral dissemination. This study demonstrates that manipulation of signaling at immune cell contacts by HIV-1 is essential for promoting virus replication and defines a paradigm for antigen-independent T cell signaling.

The molecular mechanism of substrate engagement and immunosuppressant inhibition of calcineurin.

Author SummaryTransplantation medicine was revolutionized by the introduction of the immunosuppressant drugs cyclosporin A and FK506 that prevent rejection of transplanted organs by the recipient's immune system. These drugs work by inhibiting calcineurin, a conserved protein phosphatase. Calcineurin regulates the immune response by dephosphorylating and activating the members of the NFAT family of transcription factors, which in turn activate genes required for the antigen-dependent stimulation of T-cells. Despite its biological and clinical importance, we have only a limited understanding of how calcineurin and other protein phosphatases interact with their substrates and target specific phosphorylated residues for dephosphorylation. Here, we determined the structure of calcineurin in complex with A238L, a viral peptide inhibitor of its function. This study shows that the viral peptide inhibits calcineurin not by targeting its active site but rather by occupying two critical substrate-binding regions of calcineurin (distant from each other and from the active site), thereby preventing its interaction with protein substrates. These findings allow us to present the first computational model of calcineurin bound to a phospho-substrate at its active site. Furthermore, by elucidating the structural basis for one particular mode of substrate–calcineurin interaction, this study reveals that both this viral peptide and immunosuppressant drugs inhibit calcineurin by blocking substrate access to a single critical region of the enzyme.

Genomewide Detection of Genes Targeted by Aberrant Somatic Hypermutation in Lymphoma,

Abstract 3474The processes of somatic hypermutation (SHM) and class switch recombination introduced by activation-induced deaminase (AID) are key steps for creating a pool of diversified antibodies in activated germinal center (GC) B cells (centroblasts; CBs) upon antigen dependent stimulation of resting naïve B cells (NBs) by T cells. Periodically, centroblasts enter the light zone of the GC where they interact with dendritic cells and T helper cells to undergo clonal selection for cells expressing high affinity antibodies. Unfortunately, AID can also accidentally introduce mutations at bystander loci, particularly within the promoter regions of a number of proto-oncogenes (BCL6, MYC, PAX5, PIM1, RhoH, S1PR2, and SOCS1) in diffuse large B cell lymphomas (DLBCL), an aggressive non-Hodgkins lymphomas. Since current methods, such as Exon Capture and RNAseq, only target mutations in coding regions, to date aberrant promoter SHMs have been studied only in a handful of genes. Given the complexity of lymphomagenesis, there may be additional genes that are targeted aberrantly by SHM at their promoters in DLBCLs.To identify genes that are targeted by SHM genomewide, we designed a novel approach to search for SHM hotspots in the genome by integrating bioinformatics tools with next generation sequencing technology. We developed a comprehensive software framework (called SHMseeqer) for discovery of single nucleotide variants (SNV) that have SHM features from ChIP-seq reads. We ran SHMseeqer on several histone H3 lysine 4 trimethylation (H3K4me3, preferentially enriched at promoters) ChIP-seq datasets obtained from two DLBCL cell lines and from normal primary NBs and CBs. We observed increased numbers of genes that harbor SHM at their promoters in DLBCL cells as compared to primary normal B cells. Moreover, in DLBCL cells, we observed promoter SHM hotspots in genes that have not been reported before, including BACH2, BTG2, EBF1, and TCL1A, etc., all of which have potential roles in B cell survival, differentiation, and malignant transformation. In addition, using BCL6 and BACH2 promoters as examples, we demonstrated that SHMs identified in these promoters greatly altered the promoter activities in a reporter assay. BACH2 is a B-cell specific transcription factor that plays an important role in gene expression regulation during B-cell activation and terminal differentiation. We observed that the SHM containing BACH2 promoter had increased transcription activity in the reporter assay as compared to widetype BACH2 promoter. We further showed that there was an elevated BACH2 level in DLBCL cells, and together with BCL6, BACH2 can block mature B-cell differentiation by repressing the expression of PRDM1.Our approach provides the first cost-efficient, genome-wide method to screen promoter mutations and SHM hotspots; an area that has not been fully explored thus far. Furthermore, by using this approach we showed that there were increased aberrant SHMs in DLBCL cells, suggesting that the process of AID-induced SHM is impaired in DLBCL cells. Moreover, the AID-induced promoter SHMs may deregulate the expression of key B-cell genes that leads to DLBCL transformation ultimately. Disclosures:No relevant conflicts of interest to declare.

SIRT1 links CIITA deacetylation to MHC II activation

Antigen-dependent stimulation of T cells plays a critical role in adaptive immunity and host defense. Activation of major histocompatibility complex II (MHC II) molecules, dictated by Class II transactivator (CIITA), is considered a pivotal step in this process. The mechanism underlying differential regulation of CIITA activity by the post-translational modification machinery (PTM) and its implications are not clearly appreciated. Here, we report that SIRT1, a type III deacetylase, interacts with and deacetylates CIITA. SIRT1 activation augments MHC II transcription by shielding CIITA from proteasomal degradation and promoting nuclear accumulation and target binding of CIITA. In contrast, depletion of SIRT1 upregulates CIITA acetylation and attenuates its activity. Nicotinamide phosphoribosyltransferase (NAMPT) that synthesizes NAD+ required for SIRT1 activation exerts similar effects on CIITA activity. Two different types of stress stimuli, hypobaric hypoxia and oxidized low-density lipoprotein (oxLDL), induce the acetylation of CIITA and suppress its activity by inhibiting the SIRT1 expression and activity. Thus, our data link SIRT1-mediated deacetylation of CIITA to MHC II transactivation in macrophages and highlight a novel strategy stress cues may employ to manipulate host adaptive immune system.

Differential Dephosphorylation of the FcRγ Immunoreceptor Tyrosine-based Activation Motif Tyrosines with Dissimilar Potential for Activating Syk

The cell surface-expressed gamma chain of the high affinity receptor for IgE (FcepsilonRI) can be phosphorylated on two tyrosine residues of the immunoreceptor tyrosine-based activation motif (ITAM), leading to recruitment and activation of spleen tyrosine kinase (Syk), a kinase that is essential for mast cell signaling and allergic responses. However, it is not known whether preferential phosphorylation or dephosphorylation of the two individual FcRgamma tyrosines (the N-terminal Tyr47 and C-terminal Tyr58) could regulate Syk activation. Herein we report that phosphorylation of only Tyr58 was able to elicit Syk phosphorylation and a weak rise in intracellular calcium, suggesting that Tyr58 phosphorylation may be distinctively important for Syk activation. In vitro and in vivo studies revealed that both Tyr47 and Tyr58 could be similarly phosphorylated. However, mass spectrometric analysis of the phosphorylated FcepsilonRgamma from bone marrow-derived mast cells showed that phosphorylation at Tyr47 was at least 2-fold greater than at Tyr58. This suggested that, once phosphorylated, Tyr58 is preferentially dephosphorylated. In vitro studies demonstrated more efficient dephosphorylation of Tyr58 (by the receptor-associated phosphatases SHP-1 and SHP-2) than of Tyr47. Analysis of Syk binding to wild type and mutant phosphorylated FcepsilonRI revealed that mutation at Tyr58 almost completely ablated Syk binding, whereas mutation at Tyr47 moderately reduced Syk binding. The findings argue for a novel regulatory mechanism, where dephosphorylation of phospho-Tyr58 is likely to promote the down-regulation of Syk activation and suppression of mast cell responses.

CD28:CD80 interactions mediate anitgen independent ring junction formation and signalling in T cells (87.15)

Abstract The two-signal model of T cell activation has been focused on what costimulatory signals are necessary to facilitate and enhance T cell response to a specific antigen. However, it is also intriguing to study the ability for costimulatory interactions to induce signaling in the absence of specific antigen recognition. We examine the signaling of CD4+ T cells engaging high densities of CD80 molecules in the presence of ICAM-1 in an antigen-independent T cell response. Examination of freshly isolated naive mouse T cells interacting with CD80 and ICAM-1 reveals a novel role for CD28:CD80 interactions in antigen independent ring junction formation in which CD80:CD28 interactions were located in the central region. The unengaged T-cell receptor is also localized to this central region. These cells exhibit increased cytoplasmic Ca++ compared to cells interacting with ICAM-1 alone. With as many cells fluxing Calcium as seen in antigen dependent stimulation. Mouse T cells interacting with ICAM-1 and CD80 also show an increased level of phosphorylated tyrosine residues. Although there is signaling in an antigen independent T cell response, it is not sufficient for inducing IL-2 secretion. The higher expression levels of CD80 achieved in the bilayer distinguish these results from earlier studies (Bromley et al, 2001; Markiewicz et al 2005) showing no interaction of mouse CD28 and CD80 in the absence of antigen. Thus, CD28, as well as NKG2D (Somersalo et al, 2004; Markiewicz, 2005), can stimulation formation of antigen independent ring junctions. These experiments also demonstrate that CD4+ T cells can be induced to form antigen independent adhesion rings under conditions of high CD80 density. The function of these structures is under investigation. NIH AI44931

Polymicrobial sepsis induces divergent effects on splenic and peritoneal dendritic cell function in mice.

Dendritic cells (DCs) are professional antigen-presenting cells that act as sentinels in the cell-mediated response against invading pathogens associated with septic challenge. The purpose of the present study was to determine whether there is a loss of dendritic cells and/or changes in function of these cells in septic mice. Here we report that the number of DCs, in both spleen and peritoneum, decreased over 24 h postsepsis [cecal ligation and puncture (CLP)] when compared with sham. The most dramatic change was seen in the peritoneal cavity. This decrease appeared to be caused mainly by the depletion of immature DCs rather than mature DCs. This change was LPS independent and minimally affected by FasL; however, overexpression of human Bcl-2 gene provides protection of the septic peritoneal DCs. Moreover, although the level of IL-12 release decreased significantly in splenic DCs obtained from CLP mice, IL-12 secretion was markedly elevated by peritoneal DCs as well as in both plasma and peritoneal fluid at 24 h post-CLP. In peritoneal cells, the expression of CD40, CD80, and CD86 was unchanged, but their respective ligands CD40L, CD28, and CD152 all increased in mice 24 h after CLP, although no such change was observed in splenocytes. Regardless of the presence or absence of antigen, peritoneal DCs from CLP mice showed higher capacity to stimulate T-cell proliferation than those cells from the sham control. However, splenic DCs from CLP mice only showed augmented capacity to induce antigen-dependent stimulation of T-cell proliferation. Together, these data indicate that sepsis produces divergent functional changes in splenic and peritoneal DC populations.

Cell-growth control by monomeric antigen: the cell surface expression of lysozyme-specific Ig V-domains fused to truncated Epo receptor

Previously we have shown that the V H and V L fragments of an anti-hen egg lysozyme (HEL) antibody HyHEL-10 are weakly associated but can be driven together by antigen. By joining these antibody variable domains to the cytoplasmic portion of the murine erythropoietin receptor, we created a chimeric growth factor receptor that could be activated by HEL. After co-transfection with two plasmids encoding the respective chimeric receptors in IL-3 dependent murine pro-B Ba/F3 cells, a portion of the cells survived under antigen dependent stimulation without IL-3. These surviving cells all showed coexpression of the two chimeric receptor chains and demonstrated HEL dose-dependent growth stimulation without IL-3. When another IL-3 dependent cell line 32D was transfected with a variant of such chimeric receptor with a linker peptide (Gly–Ser–Gly) inserted between V H/V L and EpoR domains, an improved growth response was attained. These observations suggest the utility of heterodimeric Fv chimeric receptors in creating cells that respond to monomeric antigen.

Endothelial Antigen Presentation: Stimulation of Previously Activated but Not Naı̈ve TCR-Transgenic Mouse T Cells

In vitroexperiments have shown that endothelial cells can function as antigen-presenting cells to CD4+T lymphocytes. The studies presented here address the question of whether naı̈ve versus previously activated CD4+helper T cells differ in their responses to endothelial antigen presentation. TCR-transgenic mice were used as a source of naive T cells of defined antigen specificity. These cells were stimulatedin vitrowith antigen and splenic antigen-presenting cells to generate populations of T lymphocytes with a previously activated/memory phenotype. Two different types of mouse endothelial cells were used as antigen-presenting cells, including the SVEC4-10 line derived from lymph node endothelium and primary murine pulmonary microvascular endothelium. Monolayer cultures of both types of endothelium were capable of antigen-dependent stimulation of previously activated TCR-transgenic CD4+cells. In contrast, neither endothelial type could activate naı̈ve CD4+T cells. When costimulatory signals were providedin transby the addition of MHC-mismatched mouse spleen cells, activation of naı̈ve T cells by endothelial antigen presentation could be demonstrated. The expression of ICAM-1 or VCAM-1 on the endothelial cells was not sufficient to activate naı̈ve T cells. Furthermore, the mouse lung endothelium constitutively expresses B7-1, and therefore, the inability of endothelium to stimulate naı̈ve T cells could not be attributed to a lack of CD28-ligands. These studies suggest that the potential role of endothelial antigen presentation in immune responses is restricted to promoting responses by T cells which have previously encountered antigen presented by other antigen-presenting cells.

Spontaneous degranulation of cultured bone-marrow-derived immature mast cells from X-linked immunodeficient (Xid) mice.

IgE-mediated, antigen-dependent stimulation of immature mouse mast cells cultured in IL-3-containing media produces secretion by granule exocytosis. Similar cultured mast cells were derived from X-linked immunodeficient (Xid) mice and examined by electron microscopy. In these cultures, Xid mast cells were also immature. In contrast to cultures obtained from control mice, 10-20% of the immature mast cells of Xid origin were undergoing secretion by granule extrusion in the absence of any secretogogue. Spontaneous secretion may be related to discordered tyrosine kinase function and/or signal transduction pathways in the Xid mouse.

Lymphocyte-activation gene 3/major histocompatibility complex class II interaction modulates the antigenic response of CD4+ T lymphocytes.

The activation requirements for antigen-dependent proliferation of CD4+ T cells are well documented, while the events leading to the inactivation phase are poorly understood. Here, we tested the hypothesis that the lymphocyte-activation gene 3 (LAG-3), a second major histocompatibility complex (MHC) class II ligand, plays a regulatory role in CD4+ T lymphocyte activation. CD4+ class II-restricted T cell clones were stimulated by their relevant antigen (hemagglutinin peptide or diphteria toxoid) and antigen-presenting cells with or without anti-LAG-3 monoclonal antibody (mAb). Kinetic studies were performed to monitor different activation parameters, including the measurement of thymidine incorporation, expression of activation antigens and cytokine secretion. Results showed that the time course from the initial time points up to the peak time point was not modified in the presence of anti-LAG-3 mAb. However, addition of these antibodies, either as whole IgG or as Fab fragments, led to increased thymidine incorporation values for late time points and, hence, to a shift in the decreasing proliferation curve. We also showed that expression of activation antigens, such as CD25, was higher in the presence of anti-LAG-3 mAb, and that cytokine concentrations, i.e. of interferon-gamma or interleukin-4, were higher in the corresponding culture supernatants. In addition, we tested whether the effects of anti-LAG-3 mAb were limited to antigen-dependent, MHC class II-restricted responses. The proliferative responses of CD4+ T cell clones following stimulation with either interleukin-2, mitogens, a combination of anti-CD2 mAb, immobilized anti-CD3 or anti-T cell receptor mAb were not altered by anti-LAG-3 mAb. The allogeneic proliferative response of a CD8+ T cell clone was also not affected. Overall, the present analysis reveals a modulating effect of anti-LAG-3 mAb, mediated specifically on antigen-dependent, MHC class II-restricted responses of CD4+ T cell lines. These results support the view that LAG-3/MHC class II interaction down-regulates antigen-dependent stimulation of CD4+ T lymphocytes.

Antigen-dependent stimulation by bone marrow-derived mast cells of MHC class II-restricted T cell hybridoma.

This paper describes a new role for mast cells as being able to present Ag to immune T cells. A mouse bone marrow-derived mast cell population obtained after 3 wk of culture in a conditioned medium has been shown to express a variety of membrane-associated Ag, including MHC class II and class I Ag, CD23, CD32, high affinity receptor for IgE, and CD4. Expression of MHC class II molecules was up-regulated upon stimulation with LPS but not with IFN-gamma and was down-regulated after exposure of mast cells to IL-3 treatment. We have demonstrated that mast cells were able to present native Ag as well as immunogenic peptides to MHC class II-restricted T cell hybridoma. The inhibition of Ag presentation after mast cells have been treated with ammonia suggests that Ag catabolism in intracytoplasmic compartment as a key step in Ag handling takes place in these cells. The MHC class II molecule is the restricting element for the presentation of OVA and the lambda repressor from bacteriophage lambda to a panel of specific T cell hybridomas, as demonstrated by the blocking effect of anti-MHC class II mAb on the Ag-presenting function. A characteristic feature of mast cells is the generation of a narrower immunogenic peptide repertoire as compared with A20 and LBB 3.4.16, a B lymphoma cell line, and a B cell hybridoma, respectively. This novel function of mast cells brings to a much closer connection inflammatory and immunologic processes and sheds new light on the biology of mast cells and particularly on the specific allergic responses.

Lithium potentiates antigen-dependent stimulation of lymphocytes only under suboptimal conditions

Immunization of hamsters with DNP-BSA in either Freund's complete or incomplete adjuvant led to the induction of antigen reactive lymph node cells. As assessed by in vitro lymphocyte stimulation assays, antigen in complete adjuvant was more effective than antigen in incomplete adjuvant in inducing immunity. Supplementing antigen-stimulated cultures from animals 14 days post-immunization with LiCl led to no enhancement of tritiated thymidine incorporation into cells from animals immunized with antigen in complete adjuvant, but did enhance antigen-dependent stimulation of cells from animals immunized with antigen + incomplete adjuvant. LiCl was, however, able to enhance stimulation of cells from animals immunized with antigen + complete adjuvant at 22 and 29 days post-immunization, when in vitro responsiveness was declining. Lymph node cells from animals optimally immunized with adjuvant were fractionated by passage over Sephadex G-10 columns. Sephadex G-10 non-adherent cells, deficient in cells such as macrophages, exhibited a depressed responsiveness to antigen, compared to unfractionated cells, and responsiveness was not restored by LiCl. Stimulation of cells by antigen was found to be inhibited by supplementing the cultures with theophylline or dibutyryl cyclic AMP and this inhibition could be reversed by LiCl. Lithium would, therefore, appear to be able to influence lymphocyte adenylate cyclase. Thus, LiCl can exert an immunopharmacologic effect on in vitro antigen stimulation primarily when conditions are suboptimal, possibly through an influence on cyclic AMP metabolism.

Tumor escape from immune elimination

A simple kinetic model is proposed for the interaction between a tumor and the immune system. Special attention is given to the phenomenon of “sneaking through” and the associated phenomena of tumor regression and recurrence. Sneaking through refers to the situations in which small antigenic tumors grow progressively, medium-sized tumors are rejected and large ones break through again. The combination of two factors is proposed as being essential for explaining this behavior: (a) The dependence of the immune response on antigen dose. (b) The negative intervention of immunosuppressors, or inhibitory factors. The immune response is described here as a repeated antigen-dependent stimulation of lymphoid cells to proliferate, with a parallel process of antigen-dependent differentiation to a terminal phase. For too small or too large antigen doses the growth in the number of cells is counter-balanced by loss from the proliferative pool through natural decay or by enhanced differentiation, respectively. When this is combined with the blocking of resting and proliferating precursor cells by factors originating from tumor cells, computer simulations demonstrate that the model is able to account for the observed patterns of tumor behavior. The model allows for a discussion of the significance of various biological parameters and is amenable for testing.

Requirement for antigen in lipopolysaccharidedependent induction of B cells

The magnitude of the IgM response to a variety of antigens, induced on the in vivo administration of lipopolysaccharide (LPS) to mice, is approximately proportional to the magnitude of the background response observed in untreated animals. This striking correlation suggests that the administration of LPS alone cannot in general induce a specific response in the absence of a background response. Such is the case for the antigen rat erythrocytes against which no LPS-dependent or background response is found. The response to a marginally immunogenic dose of rat erythrocytes, however, can be considerably enhanced by the administration of LPS. These observations are expected on the hypothesis that both background and LPS-induced responses are due to ongoing antigen-dependent stimulation in normal mice. This hypothesis is further supported by evidence suggesting that the LPS-dependent anti-sheep erythrocyte response is due, at least in part, to a particular antigen present on degraded mouse erythrocytes.

Antigen-dependent stimulation of synthesis of deoxyribonucleic acid in spleen cells from immunized rabbits.

Antigen-dependent Stimulation of Synthesis of Deoxyribonucleic Acid in Spleen Cells from Immunized Rabbits