Bone Marrow-derived Antigen Research Articles

Coagulation factor XII induces pro-inflammatory cytokine responses in macrophages and promotes atherosclerosis in mice.

Atherosclerosis is considered a chronic inflammatory disease of the vessel wall. Coagulation pathways and immune responses contribute to disease development. The role of coagulation factor XII (FXII) in vascular inflammation, however, remains controversial. We here investigated the function of FXII in atherosclerosis using apolipoprotein E and FXII-deficient (F12-/-Apoe-/-) mice. Compared to F12+/+Apoe-/- controls, atherosclerotic lesion formation was reduced in F12-/-Apoe-/- mice. This was associated with a decrease in serum interleukin (IL)-1β and IL-12 levels and reduced expression of pro-inflammatory cytokines in the aorta in atherosclerotic F12-/-Apoe-/- mice, as well as diminished Th1-cell differentiation in the aorta, blood, and lymphoid organs. No changes in circulating bradykinin, thrombin-antithrombin-complexes or plasminogen were observed. Mechanistically, activated FXII (FXIIa) was revealed to directly induce bone marrow-derived macrophages to secrete pro-inflammatory cytokines, including tumour necrosis factor-α, IL-1β, IL-12, and IL-6. Exposure of bone marrow-derived antigen presenting cells to FXIIa similarly induced pro-inflammatory cytokines, and an enhanced capacity to trigger antigen-specific interferon γ-production in CD4+ T cells. Notably, bone-marrow derived macrophages were capable of directly activating FXII. Moreover, the induction of cytokine expression by FXIIa in macrophages occurred independently of FXII protease enzymatic activity and was decreased upon phospholipase C treatment, suggesting urokinase-type plasminogen activator receptor (uPAR) to confer FXIIa-induced cell signalling. These data reveal FXII to play an important role in atherosclerotic lesion formation by functioning as a strong inducer of pro-inflammatory cytokines in antigen-presenting cells. Targeting of FXII may thus be a promising approach for treating cardiovascular disease.

Induced immune tolerance of autoantigen loaded immature dendritic cells in homogenic lupus mice

This study investigated the induced immune tolerance of autoantigen dendritic cells (imDCs) in homogenic lupus mice to support the use of dendritic cell treatment against autoimmune diseases, such as systemic lupus erythematosus. A lupus mouse was used to model based on in vitro cell culture. An immunohistochemistry assay was used to assess CD8(+), CD4(+) cell ratio in mouse spleen cells. The ratio of CD4(+)CD25(+) cells in mouse spleen lymphocytes was detected by flow cytometry, whereas the kidney was directly measured by immunofluorescence. After the injection of mouse antigen loaded bone marrow-derived antigen imDCs with a homogenetic background, mouse nucleoprotein immune with a homogenetic background was carried out. The results were compared against the simple mouse nucleoprotein immune model with a homogenic background. The 24-h urine protein, serum antinuclear antibody and anti-ds-DNA antibodies of the simple mouse model were lower compared to group S1. The CD4(+)CD25(+) cell percentage of spleen was higher in the simple mouse model compared to group S1. In the spleen, the number of lymphocyte CD8(+) cells declined, whereas the number of CD4(+) cells increased. In conclusion, after autoantigen uptake, imDCs are able to induce immune tolerance to the antigen by reinfusion, which appears to prevent or mitigate systemic lupus erythematosus-like illness.

Rapamycin-conditioned, alloantigen-pulsed myeloid dendritic cells present donor MHC class I/peptide via the semi-direct pathway and inhibit survival of antigen-specific CD8 + T cells in vitro and in vivo

Dendritic cells (DC) are “professional” bone marrow-derived antigen (Ag)-presenting cells of interest both as therapeutic targets and potential cellular vaccines due to their ability to regulate innate and adaptive immunity. Harnessing the inherent tolerogenicity of DC is a promising and incompletely explored approach to the prevention of allograft rejection. Previously, we and others have reported the ability of pharmacologically-modified DC, that resist maturation, to inhibit CD4 + T cell responses and prolong allograft survival. Here we evaluated the ability of murine myeloid DC conditioned with the immunosuppressive pro-drug rapamycin (RAPA) to acquire and directly present alloAg to syngeneic CD8 + T cells. RAPA-conditioned DC (RAPA-DC) pulsed with allogeneic splenocyte lysate acquired and expressed donor MHC class I and enhanced the apoptotic death of directly-reactive donor Ag-specific CD8 + T cells in vitro. Moreover, following their adoptive transfer, they reduced the survival of these T cells in vivo. The ability of RAPA-DC to inhibit the survival of alloAg-specific CD8 + T cells provides a potential mechanism by which host-derived DC may act as negative regulators of T cell alloreactivity and support donor-specific unresponsiveness. Adoptive cell therapy with alloAg-pulsed RAPA-DC may offer an effective approach to suppression of alloimmunity, with reduced dependence on systemic immunosuppression.

Bone marrow-derived cells present MHC class I-restricted tumour antigens in priming of antitumour immune responses.

Many tumours express tumour-specific antigens capable of being presented to CD8+ T cells by major histocompatibility complex (MHC) class I molecules. Current models of antigen presentation predict that the tumour cell itself should present its own MHC class I-restricted antigens to T cells. Earlier cross-priming experiments have demonstrated that at least some MHC class I-restricted antigens may also be presented by bystander cells. There is no detectable presentation of MHC class I-restricted tumour antigens by the tumour itself during priming of tumour-specific responses. The tumour antigens are presented exclusively by host bone marrow-derived cells. These results imply that an efficient mechanism exists in vivo for transfer of MHC I-restricted antigens to bone marrow-derived antigen presenting cells. They also suggest that HLA matching may not be critical in the clinical application of allogeneic tumour vaccines.

Treatment of Antigen-Presenting Cells with Histone Deacetylase Inhibitors Represents a Novel Strategy To Overcome CD4+ T-Cell Tolerance: Divergent Effects on the IL10 and IL-12 Promoter.

Tumor-antigen-specific T-cell tolerance imposes a significant barrier to the development of effective therapeutic cancer vaccines. Bone marrow-derived antigen presenting cells (APCs) are critical in the induction of this unresponsive state. The requirement for APCs in tolerance induction, together with their well-known role in priming T-cell antitumor responses place APCs at the crossroads of immune activation versus immune tolerance and points to manipulation of these cells as an enticing strategy to modulate T-cell responses against tumors. Identification of the intracellular mechanisms by which APCs induces either T-cell outcome represents therefore a critical step to better understand and overcome tumor-induced immune tolerance. Histones tail plays an important role in modulation of gene transcription. Emerging evidence suggest that inhibition of hystone deacetylases (HDAC) increases the expression of inflammatory genes. Given that the inflammatory status of the APC at the time of antigen presentation is central in determining T-cell priming versus T-cell tolerance, we evaluated the effects of the HDAC inhibitor LAQ842 (Novartis Pharmaceutical Inc.) on APC function and regulation of antigen-specific CD4+ T-cell responses. First, treatment of peritoneal elicited macrophages (PEM) or bone marrow derided dendritic cells (DCs) with increasing concentrations of LAQ842 resulted in enhanced acetylation of hystones H-2A, H-2B, H3 and H4. Analysis of the expression of MHC class molecules and co-stimulatory molecules revealed a significant increase in B7.2 and CD40 in LAQ842-treated APCs as compared to untreated APCs. Utilizing multi-template RNA probes and ELISA we found that LAQ842-treated APCs produce enhanced levels of several inflammatory mediators such as IL-1a, IL-1b, IL-6, TNF-a and RANTES relative to untreated APCs. Similarly, in response to LPS-stimulation, LAQ842-treated APCs produce significant higher levels of the pro-inflammatory cytokine IL-12 but reduce production of the anti-inflammatory cytokine IL-10 as determined by RT-PCR and ELISA. Furthermore, by chromatin immune precipitation (CHIP) assays we found that LAQ842-treated APCs display an increased acetylation of histones associated with the IL-12 promoter but a diminished acetylation of histones at the IL-10 promoter in response to LPS stimulation. Next, we evaluated whether the inflammatory APCs induced by LAQ842 were capable of effectively present antigen and prime productive antigen-specific T-cell responses. In vitro treatment of PEM or DCs with increasing concentrations of LAQ842 resulted in an enhanced presentation of HA-peptide to naïve CD4+ T cells specific for a MHC class II restricted epitope of influenza hemagglutinin (HA). Indeed, these clonotypic T cells display an enhanced HA-specific proliferation, IL-2 and IFN-gamma production relative to clonotypic T cells that encountered HA-antigen on untreated APCs. More importantly, LAQ842-treated APCs were able to restore the responsiveness of tolerant CD4+ T-cells isolated from lymphoma bearing hosts.By demonstrating that HDAC inhibitor induces inflammatory APCs capable of restoring the responsiveness of tolerant T-cells, our studies have unveiled a previously unknown immunological effect of these agents and have broadened their clinical scope as promising adjuvants in cancer immunotherapy.

Disruption of STAT3 Signaling in Antigen Presenting Cells Represents a Novel Strategy To Overcome Tumor-Induced Antigen-Specific CD4+ T-Cell Tolerance.

Tumor antigen-specific T-cell tolerance imposes a significant barrier to the development of effective therapeutic cancer vaccines. Bone marrow-derived antigen presenting cells (APCs) are critical in the induction of this unresponsive state. Recently, we have identified STAT3 signaling in APCs as an important regulatory pathway that determines the functional outcome of antigen-specific CD4+ T-cells in response to cognate antigen. Indeed, while disruption of this signaling pathway in APCs led to effective T cell priming, enhanced STAT3 activity resulted in the induction of T cell unresponsiveness1.Given the above results, we explored in this study whether disruption of STAT3 signaling in APCs may preserve the responsiveness of antigen-specific CD4+ T-cells during the growth of a tumor that induces antigen specific T-cell tolerance. First, mice with a genetic disruption of Stat3 in macrophages, neutrophils and a sub-population of myeloid DCs (LysMcre/Stat3flox/− mice) or control C57BL/6 mice were given subcutaneously 1x106 B16 melanoma tumor cells engineered to express Ovalbumin as a model tumor antigen (B16-OVA). Four days later, naive CD4+ T-cells (1x106) specific for a MHC class II-restricted epitope of Ovalbumin (OT-II cells) were adoptively transferred intravenously into tumor bearing mice as well as into tumor-free controls. Two weeks later animals were sacrificed and antigen-specific CD4+ T-cell responses to in vitro re-stimulation with OVA-peptide were evaluated. As expected, antigen-specific T cells re-isolated from tumor-bearing C57BL/6 mice were fully tolerant (lack of HA-specific proliferation and cytokine production). In sharp contrast, anti-OVA CD4+ T-cells isolated from tumor bearing LysMcre/Stat3flox/− mice remained fully functional as determined by their capacity to proliferate and produce IL-2 and IFN-gamma in response to cognate OVA-peptide. The demonstration that tumor-induced antigen-specific CD4+ T-cell tolerance occurs in mice with an intact STAT3 signaling in APCs, but not in mice with genetic disruption of this signaling pathway, led us next to evaluate the efficacy of pharmacologic inhibitors of STAT3 in preventing and or overcoming tumor-induced T-cell tolerance. In vivo treatment of tumor bearing mice with Tyrphostin AG490 (0.5 mg/ i.p. /twice a day x 5 days), a well-known inhibitor of STAT3 signaling, also resulted in preservation of the responsiveness of tumor-antigen specific CD4+ T-cells. Furthermore, in vitro treatment of APCs with this compound led to effective priming of naive antigen-specific T cells and breaking of antigen-specific T-cell anergy. More recently, we have evaluated the efficacy of a novel STAT3 inhibitor, compound 295558, which efficiently inhibits the DNA-binding activity of STAT3. Treatment of DCs or macrophages with this specific inhibitor led to the generation of inflammatory APCs capable of restoring the responsiveness of tolerized CD4+ T-cells isolated from tumor bearing mice.Taken together, our findings establish a critical role for STAT3 signaling in the induction of tolerance to tumor antigens in vivo. Inhibition of this signaling pathway in APCs provides a novel molecular target to overcome the remarkable barrier that tolerance to tumor antigens imposes to cancer vaccination strategies.

Generation of competent bone marrow-derived antigen presenting cells from the deer mouse (Peromyscus maniculatus)

BackgroundHuman infections with Sin Nombre virus (SNV) and related New World hantaviruses often lead to hantavirus cardiopulmonary syndrome (HCPS), a sometimes fatal illness. Lungs of patients who die from HCPS exhibit cytokine-producing mononuclear infiltrates and pronounced pulmonary inflammation. Deer mice (Peromyscus maniculatus) are the principal natural hosts of SNV, in which the virus establishes life-long persistence without conspicuous pathology. Little is known about the mechanisms SNV employs to evade the immune response of deer mice, and experimental examination of this question has been difficult because of a lack of methodologies for examining such responses during infection. One such deficiency is our inability to characterize T cell responses because susceptible syngeneic deer mice are not available.ResultsTo solve this problem, we have developed an in vitro method of expanding and generating competent antigen presenting cells (APC) from deer mouse bone marrow using commercially-available house mouse (Mus musculus) granulocyte-macrophage colony stimulating factor. These cells are capable of processing and presenting soluble protein to antigen-specific autologous helper T cells in vitro. Inclusion of antigen-specific deer mouse antibody augments T cell stimulation, presumably through Fc receptor-mediated endocytosis.ConclusionsThe use of these APC has allowed us to dramatically expand deer mouse helper T cells in culture and should permit extensive characterization of T cell epitopes. Considering the evolutionary divergence between deer mice and house mice, it is probable that this method will be useful to other investigators using unconventional models of rodent-borne diseases.

Making central T-cell tolerance efficient: thymic stromal cells sample distinct self-antigen pools.

Induction and maintenance of self-tolerance in the developing and mature T-cell repertoire is mediated by multiple mechanisms operating both in the thymus (“central tolerance”) and in peripheral lymphoid and non-lymphoid organs (“peripheral tolerance”) [1,2]. The thymus is viewed as the prime site of T-cell tolerance induction to ubiquitous (e.g. house keeping genes or MHC antigens) and abundant blood-borne antigens entering the thymus via the circulation (e.g. C5 complement protein) [3–5]. Though intensely investigated, tolerance to self-antigens which are confined to specific tissues and organs often expressed by only a minor set of cells (e.g. α-cells of the pancreas), has largely remained puzzling [68]. The prevailing view that tolerance induction via negative selection in the thymus applies only to a limited set of “abundant” proteins, however, has been challenged by recent fmdings that intrathymic expression of “tissue-specific” antigens is a common occurrence. This phenomenon is termed “ectopic” or “promiscuous” gene expression. Interestingly, this ectopic expression appears to be a unique property of thymic epithelial rather than bone marrow-derived antigen presenting cells [9–17]. Here we provide evidence that confinement of this interesting property to a certain stromal cell type, in conjunction with different efficiencies to present exogenous antigens, results in sampling by distinct thymic stromal cells of largely non-overlapping self-antigen pools. We will argue that this subtle and intricate functional complementation in tolerance induction by epithelial cells and hemopoietic cells extends the scope of central T-cell tolerance to a wide range of tissue-specific self-antigens.KeywordsTolerance InductionThymic Epithelial CellThymic MedullaThymic Stromal CellMedullary Epithelial CellThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Endothelial cells as antigen-presenting cells: role in human transplant rejection.

The immunological properties of human endothelial cells suggest they perform a pivotal role in acute and chronic rejection following solid organ transplantation. In this review the basic features of acute and chronic rejection are described as are the cellular and molecular requirements for antigen presentation. Traditionally, antigen-presenting cells are considered to be bone marrow-derived cells. However, these conclusions have been derived from rodent models of allograft rejection where bone marrow-derived passenger leukocytes are the only source of donor major histocompatibility complex (MHC) class II in the grafted organ. In contrast, in humans, virtually all the microvascular and small vessel endothelial cells are 'constitutively' positive for MHC class II antigens. The phenotypic properties of human endothelial cells, their response to cytokines and their ability to stimulate resting T cells are described. Unlike bone marrow-derived antigen presenting cells (APCs), which utilise B7/CD28 interactions, human endothelial cells utilise lymphocyte function antigen 3 (LFA3)/CD2 pathways to stimulate T cells. They activate a CD45RO + B7-independent subpopulation of T cells. Their effect on allogeneic T cells is compared with other non-bone marrow-derived cells such as fibroblasts, epithelial cells and smooth muscle cells, which are unable to stimulate resting T cells. Evidence is presented suggesting that release of MHC and non-human leukocyte antigens (HLA) from endothelial cells stimulates an alloantibody and autoimmune response leading to chronic rejection.

TOLERANCE INDUCTION IN RATS DOES NOT REQUIRE GRAFT RESIDENT DONOR-TYPE ANTIGEN PRESENTING CELLS

498 Background: It has been postulated that donor-type bone marrow-derived antigen presenting cells (APCs) may be important for tolerance induction via a mechanism of posttransplant microchimerism. Our goal was to use chimeric hearts created via BMT to test whether donor-type APCs are required for the induction and maintenance of cardiac allograft tolerance in the rat. Methods: Chimeric PVG hearts were created via DA→PVG BMTs. Elimination of PVG APCs was confirmed by flow cytometry, MLR, immunohistochemistry and RT-PCR. Chimeric and naïve hearts were then used for heterotopic transplants, in which APCs were matched or mismatched to the recipient, while organ parenchyma was always mismatched to the recipient. To induce tolerance, oral cyclosporine (CyA, 20 mg/kg, ×10 days) was given following transplantation. Rejection was monitored by palpation of the graft and histological analysis was performed in a blinded fashion. Results: CyA led to permanent survival of PVG→DA heart grafts with minimal vascular changes. 5/7 chimeric PVG hearts survived long-term in untreated DA recipients, but uniformly demonstrated mild cellular rejection and cardiac allograft vasculopathy (CAV). 4/6 chimeric PVG hearts survived long-term in CyA treated recipients, and two of these long-term surviving hearts that have been examined showed focal rejection and moderate CAV. Table Animals from all experimental groups accepted second naïve PVG hearts (normal histology) without immunosuppression (Group II: 3/3, Group III: 2/2, Group IV: 3/3) and rejected third party F344 hearts (Groups II, IV: 2/2), confirming induction of antigen-specific tolerance in CyA-treated recipients.Conclusions: Donor-type bone marrow-derived APCs are not required for tolerance induction in this rat model. Furthermore, these data support recently published work by Tullius et al showing that donor-specific tolerance can develop despite the presence of vasculopathy in the first graft.

IMMUNOHISTOCHEMICAL ANALYSIS SUGGESTS THAT ACUTE REJECTION OF MURINE CARDIAC ALLOGRAFTS LACKING DONOR-TYPE APCs PROCEEDS VIA THE INDIRECT PATHWAY

110 Background: Although graft resident, bone marrow-derived antigen presenting cells (APCs) are highly immunogenic, they are not required for rejection, as chimeric BALB/c hearts bearing CBA APCs are rejected by CBA recipients, despite a modest prolongation in survival. The goal of this study was to characterize the cellular infiltrate in acutely rejecting chimeric BALB/c hearts to determine the allorecognition pathway(s) responsible. Methods: Chimeric BALB/c hearts were created via CBA→BALB/c bone marrow transplantation. Control or chimeric hearts were transplanted heterotopically into naïve CBA recipients. Immunohistochemistry (IHC) was performed using a standard avidin biotin complex immunoperoxidase technique on frozen sections obtained from rejected hearts. Antibodies against CD4, CD8, CD11b (macrophages[Mφ]), CD11c (dendritic cells[DC]) and CD45R/B220 (B cells) were used. One frozen section from each case was stained with hematoxylin and eosin to grade the rejection. Results: As in previous studies, APC replacement prolonged graft survival, but did not prevent rejection. All specimens showed acute cellular rejection with no difference in ISHLT grade between groups. Cellular infiltrates in both groups were predominantly effector CD8+ T cells and CD11b+ Mφ. Chimeric hearts showed an increase in CD11c+ DC and CD45R/B220+ B cells and CD4+ T cells (p=.01,.03, and.05 respectively, Mann-Whitney U Test). Table,TableConclusion: Chimeric hearts showed a different pattern of cellular rejection with increased numbers of recipient-type APCs (both B cells and DC) and CD4+ T cells. This supports the hypothesis that hearts depleted of donor-type APCs are rejected by the indirect allorecognition pathway.

Class I-restricted cross-presentation of exogenous self-antigens leads to deletion of autoreactive CD8(+) T cells.

In this report, we show that cross-presentation of self-antigens can lead to the peripheral deletion of autoreactive CD8(+) T cells. We had previously shown that transfer of ovalbumin (OVA)-specific CD8(+) T cells (OT-I cells) into rat insulin promoter-membrane-bound form of OVA transgenic mice, which express the model autoantigen OVA in the proximal tubular cells of the kidneys, the beta cells of the pancreas, the thymus, and the testis of male mice, led to the activation of OT-I cells in the draining lymph nodes. This was due to class I-restricted cross-presentation of exogenous OVA on a bone marrow-derived antigen presenting cell (APC) population. Here, we show that adoptively transferred or thymically derived OT-I cells activated by cross-presentation are deleted from the peripheral pool of recirculating lymphocytes. Such deletion only required antigen recognition on a bone marrow-derived population, suggesting that cells of the professional APC class may be tolerogenic under these circumstances. Our results provide a mechanism by which the immune system can induce CD8(+) T cell tolerance to autoantigens that are expressed outside the recirculation pathway of naive T cells.

Gene vaccination with naked plasmid DNA: mechanism of CTL priming.

The injection of naked plasmid DNA directly into the muscle cells of mice has been shown to induce potent humoral and cellular immune responses. The generation of a cytotoxic T lymphocyte (CTL) response after plasmid DNA injection may involve the presentation of the expressed antigen in the context of the injected myocytes' endogenous major histocompatibility (MHC)-encoded class I molecules or may use the MHC molecules of bone marrow-derived antigen presenting cells (APC) which are capable of providing co-stimulation as well. To resolve which cell type provides the specific restricting element for this method of vaccination we generated parent-->F1 bone marrow chimeras in which H-2bxd recipient mice received bone marrow that expressed only H-2b or H-2d MHC molecules. These mice were injected intramuscularly with naked plasmid DNA that encoded the nucleoprotein from the A/PR/8/34 influenza strain, which as a single antigen has epitopes for both H-2Db and H-2Kd. The resulting CTL responses were restricted to the MHC haplotype of the bone marrow alone and not to the second haplotype expressed by the recipient's myocytes. The role of somatic tissues that express protein from injected plasmids may be to serve as a reservoir for that antigen which is then transferred to the APC. Consequently, our data show that the mechanism of priming in this novel method for vaccination uses the MHC from bone marrow-derived APC, which are efficient at providing all of the necessary signals for priming the T cell.

Murine dendritic cells loaded in vitro with soluble protein prime cytotoxic T lymphocytes against tumor antigen in vivo.

The priming of an immune response against a major histocompatibility complex class I-restricted antigen expressed by nonhematopoietic cells involves the transfer of that antigen to a host bone marrow-derived antigen presenting cell (APC) for presentation to CD8+ T lymphocytes. Dendritic cells (DC), as bone marrow-derived APC, are first candidates for presentation of tumor-associated antigens (TAA). The aim of this study was to see whether DC are able to prime in vivo antigen-specific cytotoxic T lymphocytes after exposure to a soluble protein antigen in vitro. Lacking a well-defined murine TAA, we took advantage of beta-galactosidase (beta-gal)-transduced tumor cell lines as a model in which beta-gal operationally functions as TAA. For in vivo priming both a DC line, transduced or not transduced with the gene coding for murine GM-CSF, and fresh bone marrow-derived DC (bm-DC), loaded in vitro with soluble beta-gal, were used. Priming with either granulocyte macrophage colony-stimulating factor-transduced DC line or fresh bm-DC but not with untransduced DC line generated CTL able to lyse beta-gal-transfected target cells. Furthermore, GM-CSF was necessary for the DC line to efficiently present soluble beta-gal as an H-2Ld-restricted peptide to a beta-gal-specific CTL clone. Data also show that a long-lasting immunity against tumor challenge can be induced using beta-gal-pulsed bm-DC as vaccine. These results indicate that effector cells can be recruited and activated in vivo by antigen-pulsed DC, providing an efficient immune reaction against tumors.

Ocular molecules and cells that regulate immune responses in situ

Regulation of T cell-dependent immune responses is mediated in part by bone marrow-derived antigen presenting cells (APC) that (a) process and present antigens which engage the T cell receptor and (b) secrete cytokines that influence the threshold of T cell activation. The anterior chamber of the eye is lined by the corneal endothelium (which rests on a stroma and epithelium that is devoid of class II MHC + APC) and iris/ciliary body (which contain significant numbers of bone marrow-derived cells, one third of which are class II MHC +). When tested in vitro, these potential APCs fail to present antigens in a form that activates T cells. Moreover, iris/ciliary body cells actually suppress activation of T cells exposed to antigens on conventional APC. In addition, aqueous humor under normal circumstances contains factors (one of which is TGFB) that are potent inhibitors of antigen-driven T cell activation, but spare other aspects of T cell function. Evidence suggests that the bone marrow-derived cells in iris/ciliary body are the source of this factor. Thus, the anterior chamber contains powerful forces that can prevent induction and can suppress expression of T cell mediated immunity. It is proposed that these forces are responsible for immunologic privilege and anterior chamber associated immune deviation, and for suppressing pathologic proliferation and inflammation in the anterior segment of the eye.