Immunohistologic comparison between armadillo-derived leprosin and standard lepromin skin tests in leprosy patients

A comparison was made on the in situ immunological characteristics of dermal infiltrates of early (24-hour) and late (3-4 weeks) skin reactions in leprosy patients. The skin reactions were induced by armadillo-derived leprosin coupled to liposomes and standard Dharmendra lepromin. Most lymphocytes in the early reaction induced by both antigens were positive for Leu 4, Leu 3a, OKT8 and Ia like antigens indicating thereby the presence of activated T cells. The ratio of Leu 3a/OKT8+ cells were similar. In the late reaction elicited by both antigens, the lymphocytes in the granulomas were predominantly activated T lymphocytes expressing Leu 4, Leu 3a, OKT8 and Ia like antigens. Leu 3a+ cells were scattered diffusely amidst the epithelioid cells. In contrast, the OKT8+ cells were present mainly as 'a ring' in the periphery of the granuloma. A similar ratio of Leu 3a+/OKT8+ cells was observed in these granulomas. Macrophages in the granulomas expressed Ia like antigens. These observations indicate that the immunological characteristics of dermal infiltrates in the skin reaction induced by armadillo-derived leprosin coupled to liposomes and standard Dharmendra lepromin appear to be identical.     

Expression of C-type lectin receptors by subsets of dendritic cells in human skin

C-type lectins are cell surface receptors that recognize carbohydrate structures which are often part of microbial pathogens. Several of these molecules are expressed on dendritic cells and are involved in antigen uptake. Expression of C-type lectins on dendritic cells of the human skin, i.e. Langerhans cells of the epidermis and dermal dendritic cells, has been incompletely studied to date. We therefore investigated C-type lectins in situ and on dendritic cells obtained by migration from skin explants by immunofluorescence and flow cytometry. Emphasis was laid on expression patterns of DEC-205/CD205 and BDCA-2, a marker for plasmacytoid dendritic cells. Langerhans cells in situ expressed low levels of DEC-205. Expression was upregulated upon maturation in skin explant organ culture. Most dermal dendritic cells were found to be positive for DEC-205 and DC-SIGN/CD209. Few BDCA-2-expressing cells were found in most skin samples. They were located in small groups in the dermis close beneath the basement membrane. The vast majority of all types of dendritic cells in normal human skin was of immature phenotype, i.e. did not express DC-LAMP/CD208. It is concluded that normal appearing human skin harbors different subsets of dendritic cells including few scattered BDCA-2-expressing cells, presumably plasmacytoid dendritic cells, expressing variable sets of C-type lectin receptors. This may critically contribute to the capacity of the skin immune system to flexibly respond to the world of microbial pathogens.     

移植心Th细胞免疫偏离与MHC Ⅱ类抗原表达的关系

目的:探讨Th细胞免疫偏离与移植心MHC Ⅱ类抗原表达的关系。方法:建立大鼠心脏移植模型,以同系移植和无移植动物作为对照,采用逆转录PCR技术测定移植心Ⅰ、Ⅱ类细胞因子IL-2、IL-4 mRNA水平变化,用免疫组化技术和单克隆抗体测定移植心MHC Ⅱ类抗原表达。结果:IL-2 mRNA水平和MHC Ⅱ类抗原表达随着移植心急性免疫排斥病变发展而显著增加(P＜0.01), IL-4 mRNA水平则显著降低(P＜0.01),急性免疫排斥发展到一定阶段Ⅰ、Ⅱ类细胞因子水平出现偏离时MHC Ⅱ类抗原由低表达变为高表达。结论:移植心脏急性免疫排斥过程中,Th细胞免疫偏离与MHC Ⅱ类抗原表达变化有相关性,并参与促进移植心脏MHC Ⅱ类抗原的高表达。     

Epigenetics and autoimmunity

The phenotypic characteristics of resident and infiltrating cells in skin and oral mucosa of MRL/Mp-lpr/lpr (MRL-lpr), MRL/Mp + /+ (MRL- +), NZB x NZW F, (NZB/W) and Balb/c mice have been studied using monoclonal antibodies (Mabs) and immunoeroxidase staining with the Avidin-Biotin Complex method. Macroscopically evident skin lesions appeared spontaneously exclusively in aging MRL-lpr mice. Infiltration of lymphocytes was detected within the mucosa and the skin of MRL-lpr-mice. These lymphocytes expressed predominantly L3T4 and to a lesser extent Lyt-2 phenotype (T helper/inducer and T suppressor/cytotoxic subset, respectively). Class II major histocompatibility complex antigen (Ia) expression was detected on macrophages, dendritic cells and few lymphoid cells in normal oral mucosa and skin of all strains examined. However, in the oral mucosa and skin lesions of the 4 months old MRL-lpr mouse also keratinocytes expressed Ia antigens. Keratinocytic Ia expression was also detected in oral mucosa of 8 months old MRL-+ mice. The described immunopathological findings in skin and oral mucosa of old MRL-lpr mice show a number of important similarities to human SLE disease.     

Generation of phenotypically distinct macrophage-hepatoma hybrid clones

By fusion of C3H/HeJ splenic adherent mononuclear cells enriched for macrophages with HPRT-deficient C57L/J HH- hepatoma cells, we have generated six macrophage-hepatoma hybrid clones. The hybrid nature of isolated clones was demonstrated by karyotypic analysis. The hybrid clones were screened for macrophage properties by assaying the presence of two enzymes: nonspecific esterase and lysozyme. Three of six hybrids expressed higher amount of Ia antigen and less amount of FcR; the other three hybrids expressed higher amounts of Fcr, but no Ia antigen. Phagocytosis of serum-opsonized beads is positively correlated with FcR expression, while the proliferation of antigen-primed lymphocytes is only induced by antigen-pulsed hybrids expressing Ia antigen. One hybrid clone (MH3-1) secreted significantly higher level of PGE2 and also expressed Ia antigen with higher ability of antigen-presentation. The data suggest that the cell hybridization can segregate macrophage-featured phenotypes into different hybrid clones which perform distinct functions. It may facilitate the study on the relationship of macrophage functions and the relationship between the functions and defined cell structure.     

Phenotypes of immunocompetent cells and Ia antigen expression in oral mucosa and skin of autoimmune mouse strains

The phenotypic characteristics of resident and infiltrating cells in skin and oral mucosa of MRL/Mp-lpr/lpr (MRL-lpr), MRL/Mp+/+ (MRL-(+)), NZB x NZW F1 (NZB/W) and Balb/c mice have been studied using monoclonal antibodies (Mabs) and immunoperoxidase staining with the Avidin-Biotin Complex method. Macroscopically evident skin lesions appeared spontaneously exclusively in aging MRL-lpr mice. Infiltration of lymphocytes was detected within the mucosa and the skin of MRL-lpr-mice. These lymphocytes expressed predominantly L3T4 and to a lesser extent Lyt-2 phenotype (T helper/inducer and T suppressor/cytotoxic subset, respectively). Class II major histocompatibility complex antigen (Ia) expression was detected on macrophages, dendritic cells and few lymphoid cells in normal oral mucosa and skin of all strains examined. However, in the oral mucosa and skin lesions of the 4 months old MRL-lpr mouse also keratinocytes expressed Ia antigens. Keratinocytic Ia expression was also detected in oral mucosa of 8 months old MRL-(+) mice. The described immunopathological findings in skin and oral mucosa of old MRL-lpr mice show a number of important similarities to human SLE disease.     

Early induction of MHC antigens in human liver grafts. An immunohistologic study.

The present study documents major histocompatibility complex (MHC) Class I and II expression during early acute rejection of human liver grafts. Serial graft biopsies (pretransplant, time zero, and 1 week) were studied. Ten patients received azathioprine (AZA) and prednisone; the other six patients were treated with quadruple therapy (azathioprine, cyclosporine A, prednisone, and cyclophosphamide). To study the specificity of changes in MHC antigen expression, biopsies of six patients with minor or no morphologic abnormalities served as controls. In addition, phenotypes of inflammatory cells present during rejection were analyzed using a panel of monoclonal antibodies. The results show that during acute rejection expression of MHC Class I and II antigens increased significantly in the AZA-treated patients, in a pattern similar to that seen in the patients treated with quadruple therapy, showing enhanced MHC Class I expression on hepatocytes, bile duct epithelium, and sinusoidal endothelium, and Class II antigen on Kupffer cells and sinusoidal endothelium. Bile duct epithelium was consistently positive for Class II antigen; no significant difference with the nonrejection group was observed. T cells are the predominant inflammatory cells during rejection with equal quantities of CD4+ and CD8+ cells. A majority of the infiltrating T cells show expression of Class II antigen but do not react with anti-interleukin-2 receptor antibody. This may be the result of immunosuppressive therapy or a simple reflection of the temporary expression of interleukin-2 receptors during lymphocyte activation. The authors hypothesize that the induction of MHC antigens on bile duct epithelium leads to rejection whereas the expression on hepatocytes represents an epiphenomenon.     

Graft-versus-Host Disease in the Rat: Cellular Changes and Major Histocompatibility Complex Antigen Expression in the Liver

Cellular changes in the liver were studied during an acute lethal graft-versus-host (GVH) disease in relation to the expression of major histocompatibility complex (MHC) antigens on different liver cells. Screening for MHC antigen expression revealed that control livers contained very few Ia+ cells: mainly cells in the portal tract interstitium and a small percentage of the Kupffer cells. The changes during an ongoing GVH reaction could be separated into those related to the sinusoid-associated cells, including the liver parenchyma, and those related to the portal-tract-associated cells, including periportal hepatocytes. In the sinusoids an increase in the number of Kupffer cells was seen, now all expressing Ia antigens. No damage to hepatocytes or other sinusoid-associated cells was observed. It is postulated that the increase in both number and Ia expression of the Kupffer cells is most probably due to an increased phagocytic uptake of blood-borne cellular debris and is not a result of extensive damage to hepatocytes. In the portal tracts expanding infiltrates were found composed of Ia+ T cells and macrophages (ratio 2:1). These infiltrates are probably due to a local accumulation of lymphocytes and macrophages as a result of an interaction of migrating donor-type alloreactive T cells with recipient type Ia+ cells present in the portal tract interstitium, which also interfered with normal recipient lymphocyte and macrophage traffic. Damage to portal-tract-associated cells was slight and confined to bile duct epithelial cells, which now expressed Ia antigens, and to periportal hepatocytes. In conclusion, these data do not indicate that damage to liver parenchyma plays a major role in the pathogenesis of an acute GVH reaction.     

Induction of Ia antigen expression on murine islet parenchymal cells does not diminish islet allograft survival.

Aberrant Ia antigen expression has been implicated in the pathogenesis of type 1 diabetes. Ia antigen expression was induced on isolated B10.BR murine islet parenchymal cells by culturing them for 5 days with lymphokine supernatants containing interferon-gamma (IFN-gamma) or with recombinant murine IFN-gamma + recombinant tumor necrosis factor. Ia positivity was confirmed by indirect immunofluorescence. Islets cultured for 5 days without cytokines were Ia-negative. Purified B10.BR islets allografted into the portal veins of C57BL/6J mice do not reject, which allowed the authors to determine whether aberrant expression of Ia on parenchymal cells has a deleterious effect on allograft survival. Ia-positive or Ia-negative islets were transplanted via the portal vein into diabetic C57BL/6J mice. All mice remained normoglycemic until they were killed at 30 to 60 days. Well-granulated islet allografts were identified histologically in all mice. The experiment was repeated using Balb/cJ mice as donors. Purified Balb/cJ islets are rapidly rejected by C57BL/6J mice. Induction of Ia expression on Balb/cJ islets significantly improved allograft survival. These findings indicate that Ia-positive islet cells do not induce rejection in these allograft models but may actually have a protective effect.     

Changes in distribution of Ia antigen on epithelium of the jejunum and ileum in rats infected with Nippostrongylus brasiliensis

This study compared the tissue distribution and cellular expression of Ia antigen in jejunal and ileal epithelium at various stages of intestinal inflammation produced by infecting rats with the nematode parasite Nippostrongylus brasiliensis. Tissues were examined at Day 0 (control), Day 4 (early), Day 10 (acute), and Day 16 (recovering). Frozen sections were stained with the MRC OX-4 anti-Ia monoclonal antibody using an immunoperoxidase technique. Control jejunal sections demonstrated positive epithelial Ia expression mainly in the mid-regions of the villi. The stain appeared to be mostly intracellular in the supranuclear area; the basolateral membrane stained faintly. At Day 4, a greater percentage of the epithelial cells expressed Ia, including those at the tips of the villi. The Day 10 sections demonstrated no staining at all of villus enterocytes, but the crypt epithelium was Ia positive. At Day 16, the pattern of Ia expression was similar to that seen in the early infection. In the ileum, stain was present in enterocytes over most of the villus and crypt regions except in the villus-crypt junction and did not change significantly during infection. We conclude that the changes in the expression of Ia antigen by intestinal epithelium are local to the site of infection and probably occur as a consequence of the host's inflammatory response.     

Expression of tumor necrosis factor receptors in normal kidney and rejecting renal transplants.

Activation of the TNF signal transduction cascade is initiated by the interaction of TNF with either of two cell surface receptors, TNFR-1 and TNFR-2. The levels and regulation of expression of these two receptors has been extensively analyzed in cultured cells, but little is known of TNFR expression in situ. We analyzed the expression of TNFR-1 and -2 in normal human renal kidney and in renal transplants undergoing acute cellular rejection. Immunohistochemistry and immunogold electron microscopy indicated a strong expression of TNFR-1 on the endothelium of glomeruli of normal kidney. Immunogold colocalization for TNFR-1 and a marker of the trans-Golgi network (TGN-46) demonstrated TNFR-1 within the Golgi complex in endothelial cells in normal kidney, confirming our previous studies with cultured cells. TNFR-1 expression was lost in glomeruli from acutely rejecting kidney, but TNFR-1 was detected in abundance on infiltrating leukocytes in the interstitium of allografts with acute rejection. In contrast, TNFR-2 was demonstrated predominantly in epithelial cells of distal convoluted tubule (DCT) in acute rejection kidney near TNF-expressing leukocytes. TNF was absent in normal kidney, but present in rejecting allograft. TNF was found in infiltrating leukocytes and in adjacent tubular epithelial cells. In situ hybridization showed TNFR-1 mRNA within the endothelium of the glomeruli and of a few arterioles in normal kidney, whereas TNFR-2 mRNA was seen in tubular epithelial cells of the DCT in acute transplant rejection. These data reveal that there is both differential expression and regulation of the two TNF receptors in human kidney.     

The distribution,ontogeny and origin in the rat of Ia-positive cells with dendritic morphology and of Ia antigen in epithelia,with special reference to the intestine

Ia antigens were localized in cryostat sections of rat intestine and other tissues by an indirect immunoperoxidase technique using monoclonal antibodies that recognize the rat antigens homologous to the gene products of the I-A and I-E subregions of the mouse major histocompatibility complex (MHC). Two categories of Ia⁺ cells were characterized, namely epithelial cells and bone marrow-derived cells with dendritic morphology. In the small intestine Ia antigen was present in the distal 2/3 of the absorptive epithelium but absent from the bases of the villi, the crypts and the epithelium covering the Peyer's patches. The distribution in nude rats was similar, indicating that T lymphocytes are not obligatory for its expression. In ontogeny Ia antigen was absent in the epithelium of neonatal gut, appearing at about 4 weeks of age and reaching adult levels at about 6 weeks. Different rat strains showed large differences in the amount of Ia antigen expressed by villus epithelium and the traits for the level of expression were shown to map outside the MHC. The levels of expression of Ia antigen in the proximal tubules of the kidney followed that of the gut epithelium in the different strains and in both tissues was mostly intracellular. Studies with chimeras showed that the Ia antigen in epithelial cells was not acquired from bone marrow-derived cells. The second category of cell studied had a characteristic dendritic morphology and was present in large numbers in the lamina propria of the villi and in the crypts. In the Peyer's patches these cells were present both in the subepithelial dome region and within the epithelium itself. These Ia⁺ dendritic cells were present in nude rat jejunum and appeared in normal fetal gut by 18 days gestation and were also shown to migrate into antigen-free grafts of fetal gut. This suggests that they do not require stimulation from antigens, bacterial products or T lymphocytes in order to localize in the gut or to express Ia antigen. Studies with other cell surface markers suggest that the Ia⁺ cells with dendritic morphology represent a range of cell types, some with similarities to macrophages and others to nonphagocytic dendritic cells.     

Expression of nestin after renal transplantation in the rat

Chronic allograft injury (CAI) limits the long‐term success of renal transplantation. Nestin is a marker of progenitor cells, which probably contribute to its pathogenesis. We hypothesize that nestin is induced by ischemia/reperfusion injury and acute rejection, main risk factors for CAI. Syngeneic renal transplantation was performed in Lewis rats and allogeneic transplantation in the Fischer 344 to Lewis strain combination, which results in reversible acute rejection and in CAI in the long‐run. The Dark Agouti to Lewis rat strain combination was used to study fatal acute rejection. In untreated kidneys, nestin immunoreactivity was detected in glomeruli and in very few interstitial or microvascular cells. Syngeneic transplantation induced nestin expression within 4 days, which decreased until day 9 and returned to control levels on day 42. Nestin expression was strong during acute rejection and still detected during the pathogenesis of CAI on day 42. Nestin‐positive cells were identified as endothelial cells and interstitial fibroblast‐like cells co‐expressing alpha‐smooth muscle actin. A sub‐population of them expressed proliferating cell nuclear antigen. In conclusion, nestin is induced in renal grafts by ischemia/reperfusion injury and acute rejection. It is expressed by proliferating myofibroblasts and endothelial cells and probably contributes to the pathogenesis of CAI.     

The immunologic and ultrastructural characterization of the cellular infiltrate in acute cardiac allograft rejection: prevalence of cells with the natural killer (NK) phenotype

The inflammatory cell infiltrates in 15 endomyocardial biopsies serially obtained from a human cardiac allograft during a 1 1/2-year period were characterized. An indirect immunofluorescent technique with hybridoma-derived monoclonal antibodies which preferentially react with B lymphocytes (anti-Ia), mature T cells (OKT3, Leu 1), and helper (OKT4b,d) and supressor/cytotoxic (OKT8) T-cell subsets and with natural killer cells, macrophages, and granulocytes (OKM1) was used. During each of seven rejection episodes the overwhelming majority of infiltrating cells in the endomyocardial biopsy were OKM1+Ia. These cells displayed short microvilli, a moderate amount of cytoplasm, numerous mitochondria, a large amount of rough endoplasmic reticulum, Golgi, and an indented nucleus, that is, the ultrastructural features of large, granular lymphocytes. Thus, they morphologically and phenotypically resemble those lymphoid cells which have been shown to possess natural killer (NK) functions in man. Occasional Leu 1+OKT3+ cells, some of which were OKT8+, were also seen during acute rejection. In each instance following therapy and resolution of the rejection episode only rare OKM1+Ia- cells were present. At this time the majority of the cells were Leu 1+OKT3+OKT8+. Routine biopsies, performed at times without evidence of rejection, showed only reactivity for Ia antigens by the capillary endothelium. These studies demonstrate the prevalence of cells with the natural killer phenotype in this human cardiac allograft during episodes of acute graft rejection.     

In vivo administration of interleukin 1 elicits increased Ia antigen expression on B cells through the induction of interleukin 4

The effects of the in vivo administration of interleukin 1 (IL 1) on lymphocytes from lymph node and spleen were analyzed. Mice received five daily subcutaneous (s.c.) injections of various doses of human recombinant IL 1 beta. Either 1 or 7 days after IL 1 treatment, spleens, popliteal and inguinal lymph nodes were collected. Lymphadenosis and splenomegaly were observed in the IL 1-treated animals. Lymph nodes from IL 1-treated mice contained a higher percentage of B cells than controls, and B cells from IL 1-treated mice expressed dramatically increased levels of Ia antigen. Lymphadenosis and splenomegaly, as well as the changes in subset distributions and Ia expression were transient. Concomitant treatment of mice with IL 1 and anti-IL 4 monoclonal antibody suppressed IL 1 effects on B cell Ia expression, but not on the B/T cell ratio. In situ hybridization analyses revealed that IL 1 treatment induced the expression of mRNA for IL 4, interferon-gamma, and IL 2 in lymph node and spleen cells. The distribution of cells expressing the various cytokine mRNA was markedly different between the spleens and lymph nodes.     

Immunogenicity of the Mutated H-2Kbm1 Antigen(s). Test of Thyroid Graft Rejection between B6.C-H-2bm1 and C57BL/6 Mice Following Reciprocal Immunization with Normal Versus Malignant Cells

The immunogenic properties of one (or few) selected antigen(s) encoded by the mouse major histocompatibility complex was studied using the C57BL/6(B6) mouse strain and its descendant B6.C-H-2 6m1(bm1) mutant. These strains differ in a point mutation in the H-2K region. We compared the immunogenic and antigenic expression of the mutated antigen on different bm1 tissues by testing the vulnerability of these tissues to graft rejection response in B6 recipients. Previous results demonstrated that B6 and bm1 mice do not reject reciprocal thyroid transplants, despite the acute rejection of reciprocal skin grafts. Thyroid grafts were rejected, however, after presensitizing the recipients with skin graft syngeneic with the thyroid, but not after sensitization with spleen cells. In the present work we induced tumors in bm1 mice by treating them with a chemical carcinogen (3-methylcholanthrene). We found that two out of four tumors demonstrated strict strain specificity and were rejected by all mouse strains (including the B6 recipients) except by their strain of origin. All tumors were found to be sensitive to in vitro lysis by B6 anti-bm1 effector cells. HZ1-A and HZ1-B tumor cells were rejected by B6 recipient mice but could not immunize B6 mice against a subsequent bm1 thyroid graft. When testing the immunogenicity of B6 originated EL4 leukemia cells (which are fatal to B6 mice), we found that the tumor cells were rejected by bm1 recipients, but, unlike B6 skin grafts, were incapable of inducing the rejection of a subsequent B6 thyroid transplant. The results demonstrated that an H-2K molecule may exhibit different immunological properties when expressed on cells of different tissues. The different expression of the mutated antigen on different cell types, its ability to trigger T cells but not B cells responses and the potential involvement of the tissue specific differentiation molecules in the graft rejection response are discussed.     

Effect of cyclosporine A on the regulation of Ia antigen keratinocytes expression.

Since many skin diseases characterized by positive Ia keratinocytes show improvement with cyclosporine therapy, the purpose of this study was to determine whether cyclosporine A (CyA) alters the expression of Ia keratinocytes. Nude mice were injected with normal mouse serum (NMS) to induce keratinocyte expression of the Ia antigen. The injected mice were then divided into four groups: one was treated with oral CyA; the second was treated topically with CyA twice a day; the third was treated topically with olive oil; and the fourth was injected with nude mouse serum. The third and fourth groups served as Ia positive and Ia negative controls, respectively. The mice were treated during the first 10 days after the injections. On Day 10, epidermal sheets were analyzed for Ia expression. Analysis was made by an indirect immunoperoxidase staining method using monoclonal antibodies specific for Ia determinants. Quantitation of the number of Langerhans cells was analyzed on epidermal sheets using immunodiagnostic reagents, anti-MHC-Ia, and surface ectoenzyme, ATPase. A significant reduction of Ia-positive keratinocytes was noted in the oral CyA group vs topical and olive oil groups (64.9 +/- 29.9% vs 20.1 +/- 18.7%, respectively, P less than 0.01). In a second set of experiments mice were injected with NMS, but treatment was started only on Day 10 after injections, for 10 days. The results showed that CyA failed to down-regulate Ia expression. Topical and systemic CyA did not modify Langerhans cell population. The present study showed that systemic administration of CyA significantly reduced Ia induction by keratinocytes of nude mice that were injected with NMS.     

Ia expression in chronic relapsing experimental allergic encephalomyelitis induced by long-term cultured T cell lines in mice

Chronic relapsing experimental allergic encephalomyelitis was induced in SJL mice by adoptive transfer of long-term cultured T cell lines. The T cells which were activated with myelin basic protein (MBP) derived from various species, all induced chronic relapsing experimental allergic encephalomyelitis with a similar high incidence. During the relapsing stage, lymphocytes obtained from the spleen responded well to MBP and were capable of transferring experimental allergic encephalomyelitis, whereas thymus lymphocytes did not respond to MBP. There was no difference in the proliferative response of splenocytes to MBP when splenocytes were isolated either from mice with clinical relapse or from mice that did not relapse. Pathological examination revealed a transient appearance of inflammatory cells during the acute stage. Similar cell infiltrates were also observed at the relapsing stage. The I-region associated (Ia) antigens appeared on vessels and astrocytes in the acute inflammatory lesions which coincided with the appearance of inflammatory cell infiltrates. Ia antigen expression diminished with the disappearance of inflammatory cells. During the relapsing stage, the Ia antigens were also expressed on the vessels and astrocytes in the fresh lesions. Our data indicate that MBP-reactive T cells persist at least in the spleen, for a long time. They may be reactivated by certain mechanisms probably in the central nervous system associated with the Ia-antigen expression, which facilitates the effector phase again. The initial event that triggers the Ia-expression is not known as yet.