Parenchymal microglia of naïve adult C57BL/6J mice express high levels of B7.1, B7.2, and MHC class II

In this study, we addressed B7.1, B7.2, and MHC class II expression on microglia of normal adult C57BL/6J mice, which are susceptible to MOG35-55 peptide-induced experimental autoimmune encephalomyelitis (EAE). We showed that there are two distinct major populations of CD11b(+) cells in the central nervous system (CNS) of na?ve mice: CD45 low (CD45(lo); parenchymal microglia) and CD45 intermediate (CD45(int); CNS-associated macrophages). These two populations compose CNS microglia. There is a rare CD45 high (CD45(hi)) population. By contrast, splenic CD11b(+) cells (macrophages) are CD45(int) and CD45(hi), but rarely CD45(lo). CD45(lo)CD11b(+) cells constitutively express much higher levels of B7.1, B7.2, and MHC class II compared to CD45(int) CD11b(+) cells. A shift of CD11b(+) cells from CD45(lo) to CD45(int) was observed in the CNS of EAE mice. Our study provides evidence that (1) CD45(lo) and CD45(hi), but not CD45(int), could be unique markers to differentiate parenchymal microglia from infiltrating macrophages in EAE; (2) the level of CD45 expression on parenchymal microglia (CD45(lo)) was upregulated in EAE; and (3) parenchymal microglia in normal CNS could be potent APCs by expressing high levels of B7.1, B7.2, and MHC class II molecules and could therefore play an important role in inflammation and autoimmunity in the CNS.     

Intracerebral recruitment and maturation of dendritic cells in the onset and progression of experimental autoimmune encephalomyelitis

Dendritic cells (DCs) are thought to be key elements in the initiation and maintenance of autoimmune diseases. In this study, we sought evidence that DCs recruited to the central nervous system (CNS), a site that is primarily devoid of resident DCs, play a role in the effector phase and propagation of the immune response in experimental autoimmune encephalomyelitis (EAE). After immunization of SJL mice with proteolipid protein 139-151 peptide, process-bearing cells expressing the DC markers DEC-205 and CD11c appeared early in the spinal cord. During acute, chronic, and relapsing EAE, DEC-205(+) DCs expressing a lymphostimulatory phenotype (including the mature DC marker MIDC-8, major histocompatibility complex class II, CD40, and CD86 molecules) accumulated within the CNS inflammatory cell infiltrates. More prominent infiltration of the spinal cord parenchyma by mature DCs was observed in mice with relapsing disease. Macrophage inflammatory protein 3alpha, a chemokine active on DCs and lymphocytes, and its receptor CCR6 were up-regulated in the CNS during EAE. These findings suggest that intracerebral recruitment and maturation of DCs may be crucial in the local stimulation and maintenance of autoreactive immune responses, and that therapeutic strategies aimed at manipulating DC migration could be useful in the treatment of CNS autoimmune disorders.     

A CIITA-independent pathway that promotes expression of endogenous rather than exogenous peptides in immune-privileged sites

A CIITA-independent pathway of MHC class II expression has been found in the eye and the brain, both immune-privileged sites. Although corneal endothelial cells were unable to express MHC class II in response to IFN-gamma alone, these cells readily expressed MHC class II molecules via a CIITA-independent pathway when triggered by simultaneous exposure to IFN-gamma and TNF-alpha. CIITA-independent expression of MHCclass II molecules enabled corneal endothelial cells to present cytosolic, but not endosomal, ovalbumin (OVA) to OVA-primed T cells. To determine whether CIITA-independent expression of MHC class II is relevant in vivo, minor H-only-incompatible corneal allografts prepared from CIITA knockout (KO) mice, MHC class II KO mice or wild-type donors were placed in eyes of normal mice. Cornea allografts from wild-type and CIITA KO mice suffered similar rejection fates, whereas far fewer class II-deficient corneas were rejected. In addition, MHC class II-bearing macrophages were observed in cuprizone-induced inflammatory and demyelinating brain lesions of CIITA KO mice. We conclude that class II expression via the CIITA-independent pathway enhances the vulnerability to rejection of corneal grafts expressing minor antigens. The potential relevance of CIITA-independent MHC class II expression at immune-privileged sites is discussed in relation to tolerance to strong autoantigens.     

T cell expression of CIITA represses Th1 immunity

Despite the fact that major histocompatibility complex class II transactivator (CIITA) has been known to be involved in Th1/Th2 balance in addition to its major role as a master regulator for the expression of MHC class II genes, the exact role of CIITA in Th1/Th2 balance is still controversial. To investigate whether the Th1/Th2 balance could be modulated by T cell specific expression of CIITA, we generated CIITA-transgenic mice, in which the CIITA expression is controlled by the distal promoter of p56lck, resulting in constitutive expression of CIITA predominantly in peripheral T cells. Naive CD4+ T cells from CIITA-transgenic mice exhibited a low level of IFN-gamma secretion as well as impaired Th1 polarization in vitro, while IL-4 secretion was enhanced under Th2 condition. In addition, the development of experimental autoimmune encephalomyelitis (EAE), a prototype of Th1-mediated disease, was repressed in CIITA-transgenic mice. Resistance to EAE was correlated with reduced production of IFN-gamma in response to MOG35-55, while the proliferation of MOG35-55 -specific T cells was not affected in CIITA-transgenic mice. Together, these data demonstrate that overexpression of CIITA in T cells inhibits Th1 differentiation and function, suggesting that the expression of CIITA in T cells might play a role in the regulation of the Th1/Th2 balance during the T cell lineage commitment.     

Upregulation of group 1 CD1 antigen presenting molecules in guinea pigs with experimental autoimmune encephalomyelitis: an immunohistochemical study

In humans, group 1 CD1 glycoproteins present foreign and self lipid and glycolipid antigens to T-cells. Homologues of these molecules are not found in mice or rats but are present in guinea pigs (GPs). We examined CD1 and MHC class II expression in the central nervous system (CNS) of GPs sensitized for experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis. In normal GPs and the uninflamed CNS, low-level MHC class II (MHC II) immunoreactivity occurred on vascular elements, meningeal macrophages and parenchymal microglial cells, whereas immunoreactivity for CD1 was absent. In the inflamed CNS, the majority of infiltrating cells were MHC II+ and microglia showed increased expression. CD1 immunoreactivity was detected on astrocytes and subsets of inflammatory cells Including B cells and macrophages. Minimal CD1 and MHC II co-expression was noted on inflammatory cells or glia. We conclude that group 1 CD1 molecules are strongly upregulated in the inflamed CNS on subsets of cells distinct from the majority of MHC II bearing cells. The expression of CD1 proteins in such lesions broadens the potential repertoire of antigens recognized at these sites and highlights the value of the GP as a model for studies of the relevance of CD1 molecules in host defense and autoimmune diseases.     

Therapeutic potential of IFN-gamma-modified dendritic cells in acute and chronic experimental allergic encephalomyelitis

Dendritic cells (DC) are antigen-presenting cells specialized to regulate immune responses. DC not only control immunity, but also maintain tolerance to self-antigens-two complementary functions that would ensure the integrity of the organism in an environment full of pathogens. Here we report that splenic DC that had been exposed in vitro to IFN-gamma (IFN-gamma-DC) exhibit therapeutic potential on acute experimental allergic encephalomyelitis (EAE) in Lewis rats, and on chronic-relapsing EAE in B6 and SJL/J mice. During incipient EAE [day 5 post-immunization (p.i.) in rats, day 7 p.i. in mice], IFN-gamma-DC were injected s.c. Severity of clinical signs of EAE was dramatically inhibited in animals injected with IFN-gamma-DC, showing normal magnetic resonance imaging (MRI) of the spinal cord and brain. In contrast, the EAE rats receiving PBS or naive DC had severe clinical signs with multiple and extensive MRI lesions in the spinal cord and brain. IFN-gamma-DC triggered an antigen-specific IFN-gamma production, and induced apoptosis of CD4(+) T cells possibly through DC expressing indoleamine 2,3-dioxygenase and/or an IFN-gamma-dependent pathway. As a result, infiltration of macrophages and CD4(+) T cells within the spinal cords was dramatically reduced in animals injected with IFN-gamma-DC as compared to animals injected with PBS or naive DC. This approach may represent a novel possibility of individualized immunotherapy using autologous, in vitro modified DC as a complement to conventional therapy in multiple sclerosis and other diseases with an autoimmune background.     

Antitumour activity mediated by CD4+ cytotoxic T lymphocytes against MHC class II-negative mouse hepatocellular carcinoma induced by dendritic cell vaccine and interleukin-12

When BALA/c mice with BNL hepatocellular carcinoma (HCC) were treated with dendritic cells fused with BNL cells (DC/BNL) and recombinant murine interleukin (IL)-12, tumour development was significantly suppressed, whereas treatment with either DC/BNL or IL-12 alone did not show a tumour-suppressive effect. Antitumour activity induced by DC/BNL + IL-12 was abrogated by depletion of CD4+ T cells, but not by depletion of CD8+ T cells or natural killer cells. Splenic CD4+ T cells and CD8+ T cells from DC/BNL-treated mice showed cytotoxic activity against BNL cells after 3 days of incubation with DC/BNL, although BNL cells do not express major histocompatibility complex (MHC) class II molecules even after treatment with interferon (INF)-gamma. Furthermore, CD4+ T cells killed syngeneic-irrelevant CT26 cells and even allogeneic Hepa1-6 cells. This cytotoxicity was blocked by concanamycin A, but not by an anti-Fas ligand (FasL) monoclonal antibody, indicating that cytotoxic activity was mediated by perforin. Immunofluorescence microscopy demonstrated that abundant CD4+ T cells and MHC class II-positive macrophages, but not CD8(+) T cells, had infiltrated tumour tissue in mice treated with DC/BNL + IL-12. Flow cytometric analysis of tumour-infiltrating cells in mice treated with DC/BNL + IL-12 showed increases in CD4+ T cells and MHC class II+ CD11b+ cells but not in CD8+ T cells or MHC class I+ CD11b+ cells. Our results suggest that, in BNL-bearing mice treated with DC/BNL + IL-12, tumour macrophages activated by INF-gamma produced by IL-12-stimulated T cells might present BNL tumour antigens and activate DC/BNL-primed CD4+ cytotoxic T lymphocytes (CTLs) in a MHC class II-dependent manner, leading to perforin-mediated bystander killing of neighbouring MHC class II-negative tumour cells.     

Role of MHC class II expressing CD4+ T cells in proteolipid protein(91-110)-induced EAE in HLA-DR3 transgenic mice

MHC class II molecules play a central role in the control of adaptive immune responses through selection of the CD4(+) T cell repertoire in the thymus and antigen presentation in the periphery. Inherited susceptibility to autoimmune disorders such as multiple sclerosis, rheumatoid arthritis and IDDM are associated with particular MHC class II alleles. Advent of HLA transgenic mice has helped us in deciphering the role of particular HLA DR and DQ class II molecules in human autoimmune diseases. In mice, the expression of class II is restricted to professional antigen-presenting cells (APC). However, in humans, class II is also expressed on T cells, unlike murine T cells. We have developed new humanized HLA class II transgenic mice expressing class II molecules not only on APC but also on a subset of CD4(+) T cells. The expression of class II on CD4(+) T cells is inducible, and class II(+) CD4(+) T cells can present antigen in the absence of APC. Further, using EAE, a well-established animal model of MS, we tested the functional significance of these class II(+) CD4(+) T cells. DR3.AEo transgenic mice were susceptible to proteolipid protein(91-110)-induced EAE and showed CNS pathology accompanied by widespread inflammation and demyelination seen in human MS patients, suggesting a role for class II(+) CD4(+) T cells in the pathogenesis.     

Deficient arginase II expression without alteration in arginase I expression attenuated experimental autoimmune encephalomyelitis in mice

In the past there have been a multitude of studies that ardently support the role of arginase II (Arg II) in vascular and endothelial disorders; however, the regulation and function of Arg II in autoimmune diseases has thus far remained unclear. Here we report that a global Arg II null mutation in mice suppressed experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis. During EAE, both Arg I and Arg II were induced in spinal cords, but only Arg II was induced in spleens and splenic dendritic cells (DCs). DC activation by lipopolysaccharide (LPS), CD40L or TLR8 agonist significantly enhanced Arg II expression without affecting Arg I expression. Conversely, DC differentiating cytokines [IL‐4 and granulocyte macrophage‐colony‐stimulating factor (GM‐CSF)] yielded opposite effects. In addition, Arg I and Arg II were regulated differentially during Th1 and Th17 cell polarization. Arg II deficiency in mice delayed EAE onset, ameliorated clinical symptoms and reduced myelin loss, accompanied by a remarkable reduction in the EAE‐induced spinal cord expression of Th17 cell markers (IL‐17 and RORγt). The abundance of Th17 cells and IL‐23⁺ cells in relevant draining lymph nodes was significantly reduced in Arg II knockout mice. In activated DCs, Arg II deficiency significantly suppressed the expression of Th17‐differentiating cytokines IL‐23 and IL‐6. Interestingly, Arg II deficiency did not lead to any compensatory increase in Arg I expression in vivo and in vitro. In conclusion, Arg II was identified as a factor promoting EAE likely via an Arg I‐independent mechanism. Arg II may promote EAE by enhancing DC production of Th17‐differentiating cytokines. Specific inhibition of Arg II could be a potential therapy for multiple sclerosis.     

CXCL12 expression within the CNS contributes to the resistance against experimental autoimmune encephalomyelitis in Albino Oxford rats

Experimental autoimmune encephalomyelitis (EAE) is an animal model of multiple sclerosis, a chronic inflammatory and demyelinating disease of the CNS. Albino Oxford (AO) rats are resistant to the induction of EAE, while the disease can be readily induced in Dark Agouti (DA) rats. Here we investigated a potential contribution of the CNS milieu in the limitation of the encephalitogenic autoimmune response. EAE was induced by immunization of the respective rat strains with spinal cord homogenate emulsified in complete Freund's adjuvant. AO rats did not exhibit clinical signs after immunization while DA rats developed severe neurologic deficits. Infiltration of immune cells into spinal cords (SC) was evident in both strains 12-14 days after the immunization. EAE lesions of AO rats contained substantially lower numbers of CD4+ T cells and CD11b+ cells compared to those in DA rats. This went together with lower levels of interferon (IFN)-γ and interleukin (IL)-17 in the cells isolated from SC. We found a dramatic increase of CXCL12 expression in SC tissue and microvessels of AO rats, whereas DA rats markedly decreased the expression of this chemokine within their CNS. Administration of the CXCL12 antagonist AMD3100 to a substrain of AO rats that developed a weak EAE led to earlier onset and exacerbation of the disease. These results suggest a role of CXCL12 in down-regulating autoimmune processes in AO rats during EAE. Therapeutic modulation of CXCL12 could be a promising strategy for the treatment of CNS autoimmunity.     

Induction of experimental autoimmune encephalomyelitis in transgenic mice expressing ovalbumin in oligodendrocytes

We have used the 5' flanking sequence of the myelin basic protein gene known to include the core promoter and a strong oligodendrocyte (ODC)-specific enhancer to target expression of the well-studied model antigen ovalbumin (OVA) to ODC in transgenic mice. OVA protein was detected in a tissue- and cell-specific manner in these "ODC-OVA" mice. Without immunization, CD4 T cells and B cells remained ignorant of the neo-self antigen expressed in the central nervous system (CNS), as indicated by unimpaired development and lack of activation of OVA/IA(b)-specific TCR transgenic T cells in these mice, and the ability to mount normal OVA-specific recall and antibody responses. Upon immunization with OVA in complete Freund's adjuvant, about half of the transgenic mice developed neurological symptoms characteristic of experimental autoimmune encephalomyelitis (EAE). Mononuclear infiltrates in the brain and spinal cord contained both macrophages and T cells, similar to classical models of EAE induced by immunization with CNS antigens in adjuvant. The wealth of immunological reagents available to study and manipulate the OVA-specific response should make this new model useful for the investigation of components and mechanisms involved in CNS-specific autoimmunity.     

Role of cervical lymph nodes in autoimmune encephalomyelitis in the Lewis rat

Lymphocytes enter the central nervous system (CNS) in response to virus infections and in autoimmune diseases, such as multiple sclerosis (MS), but the origin of such lymphocytes is unclear. This study investigates the role of the cervical lymph nodes as a source of lymphocytes involved in experimental autoimmune disease of the brain. Acute active experimental autoimmune encephalomyelitis (EAE) is used as a model for the autoimmune aspects of MS and is characterized by lymphocyte and monocyte invasion and microglial activation, mainly in the spinal cord, 12–15 days post-inoculation (dpi) of antigen. Few lesions occur in the cerebral hemispheres in acute EAE, but a cryolesion to the surface of the brain 8 dpi results in a six-fold enhancement of cerebral EAE. The present study tests the hypothesis that cervical lymphadenectomy will reduce the enhancement of cerebral EAE induced by a cryolesion. Acute EAE was induced in 25 Lewis rats and a cryolesion to the brain, 8 dpi, in 16 rats was immediately followed by either cervical lymphadenectomy (n=8) or sham lymphadenectomy (n=8). The severity of EAE at 15 dpi, in the brain and spinal cord, was evaluated using immunocytochemistry for T lymphocytes (W3/13) and MHC class II expression (OX6). The results of the study showed that cervical lymphadenectomy reduced the level of cerebral EAE induced by a cryolesion by 40 per cent when compared with the sham-operated animals (P<0·01). This suggests that cervical lymph nodes play a pivotal role in the induction of EAE in the brain, possibly as a site for ‘priming’ T cells to target the brain. Investigation of the interrelationships between cervical lymph nodes and the brain in man may lead to new therapeutic strategies for multiple sclerosis. © 1997 John Wiley & Sons, Ltd.     

Susceptibility of mice deficient in the MHC class II transactivator to infection with Mycobacterium tuberculosis

Major histocompatibility complex (MHC) class II antigen presentation and subsequent CD4+ T-cell activation are critical for acquired immunity to Mycobacterium tuberculosis infection. MHC class II gene expression is primarily controlled by the master transactivator CIITA protein. Without functional CIITA protein, MHC class II expression is lost, impairing immune responses and increasing susceptibility to infection. In this study, we compared protective immune responses of CIITA-deficient mice and wild-type C57BL/6 controls with low dose aerosol M. tuberculosis infection. After aerogenic challenge, CIITA-/- mice failed to limit mycobacterial growth (2.5 and 2.0 log10 > WT lung and spleen CFUs, respectively, at day 58). Lung histopathology involved extensive necrosis, severe pneumonitis and overwhelming inflammation in the gene knockout mice. Mean survival time for CIITA-/- mice was significantly reduced (57 versus >300 days for WT). This extreme sensitivity to tuberculous infection was largely attributed to the absence of CD4+ cells. Flow cytometric studies detected virtually no CD4+ cells in CIITA-/- mouse spleens after infection versus elevated numbers in WT spleens. Failed CD4+ T-cell expansion markedly reduced interferon-gamma (IFN-gamma production in CIITA-/- mice versus WT controls. These results suggest the necessity of a functional CIITA pathway for controlling tuberculous infections and that interventions targeting CIITA expression may be useful antimycobacterial therapeutics.     

Immune regulatory mechanisms influence early pathology in spinal cord injury and in spontaneous autoimmune encephalomyelitis

Injuries to the central nervous system (CNS) trigger an inflammatory reaction with potentially devastating consequences. In this report we compared the characteristics of the inflammatory response on spinal cord injury (SCI) caused by a stab wound between the T7 and T9 vertebrae and spontaneous experimental autoimmune encephalomyelitis (EAE). SCI and EAE were compared in two types of myelin basic protein Ac1-11-specific T-cell receptor transgenic mice: T/R+ mice harbor regulatory T cells, and T/R- mice lack regulatory T cells. Our results show that 8 days after SCI, T/R- mice developed a strong T-cell infiltrate in the spinal cord, with remarkable down-modulation of CD4 expression that was accompanied by a local increase in Mac-3+ and F4/80+ reactivity and diffuse local and distal astrogliosis. In contrast, T/R+ mice exhibited a modest increase in CD4+ cells localized to the site of injury, without CD4 down-modulation; focal astrogliosis was restricted to the site of the lesion, although Mac-3+ and F4/80+ cells were also present. Similarly to T/R- mice that underwent SCI, T cells displaying down-modulated CD4 expression were found in the CNS of older T/R- mice afflicted by spontaneous EAE. Overall, our results suggest that common mechanisms regulate T-cell accumulation in CNS lesions of different causes, such as mechanic lesion or autoimmune-mediated damage.     

MHC class II expression is differentially regulated in plasmacytoid and conventional dendritic cells

Major histocompatibility complex (MHC) class II-restricted antigen presentation is essential for the function of dendritic cells (DCs). We show here that plasmacytoid DCs (pDCs) differ from all other DC subsets with respect to expression of CIITA, the 'master regulator' of MHC class II genes. The gene encoding CIITA is controlled by three cell type-specific promoters: pI, pIII and pIV. With gene targeting in mice, we demonstrate that pDCs rely strictly on the B cell promoter pIII, whereas macrophages and all other DCs depend on pI. The molecular mechanisms driving MHC class II expression in pDCs are thus akin to those operating in lymphoid rather than myeloid cells.     

Lymphocyte targeting of the brain in adoptive transfer cryolesion-EAE

Lymphocyte infiltration and microglial activation in experimental autoimmune encephalomyelitis (EAE) are mainly centred on the spinal cord. However, a cryolesion to one cerebral hemisphere (cryolesion-EAE) induces six-fold enhancement of EAE in the cerebral hemispheres and removal of the cervical lymph nodes reduces such enhancement by 40 per cent. This study tests the hypothesis that lymphocytes from donor rats with cryolesion-EAE will selectively target the brain rather than the spinal cord when transferred to naive recipients. Acute EAE was induced in 15 Lewis rats (donors); ten donors received a cryolesion to the left cerebral hemisphere 8 days post-inoculation of antigen and adjuvant. Five rats with EAE received no cryolesion. Lymphocytes from cryolesion-EAE donors or from EAE-only donors were cultured for 72h in medium containing myelin basic protein and then injected into a total of 21 naive recipients, which were killed 8 days later. The severity of EAE in brains and spinal cords was assessed in immunocytochemically stained sections by quantifying the number of vessels showing lymphocyte cuffs (W3/13 antibody) and the level of MHC class II antigen expression by microglia (OX6 antibody). When compared with recipients of EAE-only donor lymphocytes, the severity of cerebral EAE was increased 2- to 2·6-fold in the recipients of crylesion-EAE donor lymphocytes (p < 0·01); EAE in the spinal cord was reduced. These results suggest that lymphocytes from cryolesion-EAE donors preferentially target the brain in recipient animals in preference to the spinal cord. By analogy with cryolesion-EAE, focal central nervous system (CNS) damage with drainage of auto-antigens to regional lymph nodes in man may play a role in determining the site and timing of initial and recurrent multiple sclerosis lesions. Copyright © 1999 John Wiley & Sons, Ltd.     

γδ T cells in EAE: Early trafficking events and cytokine requirements

We have previously shown that γδ T cells traffic to the CNS during EAE with concurrently increased expression of β₂‐integrins and production of IFN‐γ and TNF‐α. To extend these studies, we transferred bioluminescent γδ T cells to WT mice and followed their movement through the acute stages of disease. We found that γδ T cells rapidly migrated to the site of myelin oligodendrocyte glycoprotein peptide injection and underwent massive expansion. Within 6 days after EAE induction, bioluminescent γδ T cells were found in the spinal cord and brain, peaking in number between days 10 and 12 and then rapidly declining by day 15. Reconstitution of γδ T cell^?/? mice with γδ T cells derived from β₂‐integrin‐deficient mice (CD11a, ‐b or ‐c) demonstrated that γδ T‐cell trafficking to the CNS during EAE is independent of this family of adhesion molecules. We also examined the role of γδ T‐cell‐produced IFN‐γ and TNF‐α in EAE and found that production of both cytokines by γδ T cells was required for full development of EAE. These results indicate that γδ T cells are critical for the development of EAE and suggest a therapeutic target in demyelinating disease.     

Tumor‐mediated down‐regulation of MHC class II in DC development is attributable to the epigenetic control of the CIITA type I promoter

In patients with cancer, DC express significantly lower amounts of MHC class II compared with those of normal individuals. However, the underlying mechanisms for this have not yet been fully defined. In the present study, we found that IL‐10 plays a major role in the tumor‐conditioned medium (TCM)‐mediated decrease of MHC class II expression on BM‐derived DC. IL‐10 inhibited the expression of type I CIITA during DC differentiation. GM‐CSF‐mediated histone (H3 and H4) acetylation at the type I promoter (pI) locus of the CIITA gene was markedly increased during DC differentiation and this increase was blocked by IL‐10. We also found that STAT5 bound to the CIITA pI locus during DC differentiation, and the binding was markedly attenuated by TCM or IL‐10. Altogether, these findings suggest that (i) the down‐regulation of MHC class II in tumor microenvironments is most likely attributable to IL‐10 in the TCM and (ii) IL‐10‐mediated MHC class II down‐regulation results from the inhibition of type I CIITA expression. This inhibition is most likely due to blocking of the STAT5‐associated epigenetic modifications of the CIITA pI locus during the entire period of DC differentiation from BM cells, as opposed to a simple inhibition of MHC class II expression at the DC stage.