An MRL/MpJ-lpr/lpr substrain with a limited expansion of lpr double-negative T cells and a reduced autoimmune syndrome

The autosomal recessive mutant gene, lpr, has been shown toaccelerate the progression of lupus-like autoimmune disease,which is associated with a massive expansion of a unique CD4^–CD8^–double-negative T cell subset, in MRL/MpJ mice. Here we reporta substrain of MRL/MpJ-lpr/lpr (MRL-lpr) mice which live almosttwice as long with delayed development of glomerulonephritis,compared with conventional MRL-lpr mice. This substrain, termedMRL-lpr.II (II for long-lived), develops generalized lymphadenopathycharacteristically seen in MRL-lpr mice. However, the expansionof a double negative lpr T cell subset is markedly limited witha mean value of 15% in their lymph nodes compared to about 70%in conventional MRL-lpr mice. Overall production of autoantibodies,such as anti-DNA and rheumatoid factors, does not significantlydiffer between the two MRL-lpr mice. However, serum levels ofcryoglobulins, whose major component is lgG3, are markedly diminishedin MRL-lpr.ll mice with a parallel decrease in lgG3. Since MRL-lpr.llmice still carry the lpr mutation, as documented by the presenceof defects in the Fas antigen, a possible new mutation in thissubstrain may play a significant role in the pathogenesls oflupus-like autoimmune syndrome.     

An analysis of apoptosis in lymphoid organs and lupus disease in murine systemic lupus erythematosus (SLE)

Apoptosis is a programmed cell death process that helps to regulate both T cell and B cell development. In this study, we have investigated the levels of apoptotic death in cells of the thymuses and spleens (white matter) of autoimmune MRL-lpr/lpr mice with progressive lymphadenopathy and SLE disease activity; we also examined the renal pathology in these animals. Fas is a cell surface receptor, which when activated initiates the sequence of events that lead to apoptosis. In MRL-lpr/lpr mice Fas is defective, so the competency for apoptosis may be reduced. In young animals of advancing age the thymuses enlarged until in 5-month-old females the average weight was three times that at 1 month, and spleen and kidney weights also increased in size disproportionately. At light microscope level apoptotic cells in tissue sections were counted using both routine eosin and haematoxylin staining (to identify them by their morphology) and in situ end-labelling of cells with DNA strand breaks; their presence was further confirmed by electron microscopy. As the mice aged, the numbers of apoptotic cells in thymic cortex, thymic medulla and spleen white pulp areas reduced significantly (P < 0·01–0·001), whereas in BALB/c normal controls they increased significantly (P < 0·05). These changes were coincident with the development of severe lupus, whose activity was assessed by measuring serum anti-ssDNA and anti-dsDNA antibody titres and urinary protein (albumin) level which were elevated significantly by 5 months of age (P < 0·001 for both ssDNA and dsDNA and P < 0·01 for urine albumin) compared with their younger counterparts. Thus, lymphoid organ enlargement, decrease in apoptotic indices, elevated serum anti-ssDNA and anti-dsDNA antibody levels, and impaired renal function coincided with the onset and severity of lupus disease in lpr mice. It seems likely that there is a causal relationship between defective deletion of autoreactive lymphoid cells, imperfect Fas-mediated apoptosis and development of murine SLE.     

Elucidation of the protein kinase C-dependent apoptosis pathway in distinct subsets of T lymphocytes in MRL-lpr/lpr mice

MRL-lpr mice are severely impaired in the Fas pathway of apoptosis induction. We here evaluate another pathway of apoptosis induction in MRL-lpr mice which is protein kinase C (PKC) dependent. Despite the defect of the Fas pathway, apoptosis developed during culture in vitro in splenic T lymphocytes from MRL-lpr mice more extensively than in T lymphocytes from MRL-+/+ mice. Apoptosis induction in the former cells was then found to be greatly promoted by PKC inhibitor H-7, and partially prevented by PKC activator phorbol 12-myristate 13-acetate (PMA). High sensitivity to H-7, but not to PKA inhibitor HA 1004, of these cells for apoptosis induction was confirmed by detailed time course and dose-dependency experiments of the drug effect. Population analysis showed that both CD4⁺ T lymphocytes and CD8⁺ T lymphocytes from MRL-lpr mice were highly sensitive to H-7, whereas CD8⁺ T lymphocytes, but not CD4⁺ T lymphocytes, from MRL-+/+ mice were susceptible to the reagent. Interestingly, B220⁺Thy-1⁺CD4^?CD8^? T lymphocytes from MRL-lpr mice were most sensitive to H-7 for apoptosis induction. Correspondingly, the membrane-translocated activated PKC-α level in splenic T lymphocytes from MRL-lpr was more extensively up-regulated by PMA than in splenic T lymphocytes from MRL-+/+. These results suggest that some signal consistently activates PKC in MRL-lpr T lymphocytes, and this event is needed for survival of these cells. On the other hand, CD4⁺CD8⁺ thymocytes were deleted by apoptosis in culture with PMA, whether these thymocytes were from MRL-lpr mice or MRL-+/+ mice. This finding suggested that the apoptosis induction pathway linked to PKC activation is intact in CD4⁺ CD8⁺ thymocytes from the Fas-defective MRL-lpr mice. We conclude from these results that the PKC-dependent signal pathways for either cell death or cell activation are intact or even accelerated in lpr mice, which could both compensate for the loss of the Fas pathway and promote the generation of autoreactive T lymphocytes.     

The role of microparticles in the generation of immune complexes in murine lupus

Systemic lupus erythematosus is a systemic inflammatory disease characterized by antibodies to nuclear molecules in association with immune complex deposition. As shown previously, microparticles (MPs), which are small membrane-bound vesicles released from dying and activated cells, contain nucleic acids and can form immune complexes found in patient blood. To assess the role of MPs in murine lupus, we used flow cytometry to measure the presence of MPs with bound IgG in the blood of MRL-lpr/lpr and NZB/W mice. These studies showed much higher numbers of MPs with bound IgG in the blood of MRL lpr/lpr compared to NZB/W mice. Furthermore, these studies showed that antibodies from MRL-lpr/lpr mice bound better to MPs from apoptotic cells than those from NZB/W mice. Together, these studies indicate important differences in the serological features of the two strains as reflected by the capacity of antibodies to bind to MPs.     

Fas (CD95/APO-1) limits the expansion of T lymphocytes in an environment of limited T-cell antigen receptor/MHC contacts

Fas-deficient mice (Fas(lpr/lpr)) and humans have profoundly dysregulated T lymphocyte homeostasis, which manifests as an accumulation of CD4(+) and CD8(+) T cells as well as an unusual population of CD4(-)CD8(-)TCRαβ(+) T cells. To date, no unifying model has explained both the increased T-cell numbers and the origin of the CD4(-)CD8(-)TCRαβ(+) T cells. As Fas(lpr/lpr) mice raised in a germ-free environment still manifest lymphadenopathy, we considered that this process is primarily driven by recurrent low-avidity TCR signaling in response to self-peptide/MHC as occurs during homeostatic proliferation. In these studies, we developed two independent systems to decrease the number of self-peptide/MHC contacts. First, expression of MHC class I was reduced in OT-I TCR transgenic mice. Although OT-I Fas(lpr/lpr) mice did not develop lymphadenopathy characteristic of Fas(lpr/lpr) mice, in the absence of MHC class I, OT-I Fas(lpr/lpr) T cells accumulated as both CD8(+) and CD4(-)CD8(-) T cells. In the second system, re-expression of β(2)m limited to thymic cortical epithelial cells of Fas(lpr/lpr) β(2)m-deficient mice yielded a model in which polyclonal CD8(+) thymocytes entered a peripheral environment devoid of MHC class I. These mice accumulated significantly greater numbers of CD4(-)CD8(-)TCRαβ(+) T cells than conventional Fas(lpr/lpr) mice. Thus, Fas shapes the peripheral T-cell repertoire by regulating the survival of a subset of T cells proliferating in response to limited self-peptide/MHC contacts.     

Toll-like receptor 7 deficiency suppresses type 1 diabetes development by modulating B-cell differentiation and function

Innate immunity mediated by Toll-like receptors (TLRs), which can recognize pathogen molecular patterns, plays a critical role in type 1 diabetes development. TLR7 is a pattern recognition receptor that senses single-stranded RNAs from viruses and host tissue cells; however, its role in type 1 diabetes development remains unclear. In our study, we discovered that Tlr7-deficient (Tlr7^−/−) nonobese diabetic (NOD) mice, a model of human type 1 diabetes, exhibited a significantly delayed onset and reduced incidence of type 1 diabetes compared with Tlr7-sufficient (Tlr7^+/+) NOD mice. Mechanistic investigations showed that Tlr7 deficiency significantly altered B-cell differentiation and immunoglobulin production. Moreover, Tlr7^−/− NOD B cells were found to suppress diabetogenic CD4⁺ T-cell responses and protect immunodeficient NOD mice from developing diabetes induced by diabetogenic T cells. In addition, we found that Tlr7 deficiency suppressed the antigen-presenting functions of B cells and inhibited cytotoxic CD8⁺ T-cell activation by downregulating the expression of both nonclassical and classical MHC class I (MHC-I) molecules on B cells. Our data suggest that TLR7 contributes to type 1 diabetes development by regulating B-cell functions and subsequent interactions with T cells. Therefore, therapeutically targeting TLR7 may prove beneficial for disease protection.     

Heat shock protein 90 inhibition by 17-DMAG lessens disease in the MRL/lpr mouse model of systemic lupus erythematosus

Elevated expression of heat shock protein 90 (HSP90) has been found in kidneys and serum of systemic lupus erythematosus (SLE) patients and MRL/Mp-Fas^lpr/Fas^lpr (MRL/lpr) autoimmune mice. We investigated if inhibition of HSP90 would reduce disease in MRL/lpr mice. In vitro, pretreatment of mesangial cells with HSP90 inhibitor Geldanamycin prior to immune-stimulation showed reduced expression of IL-6, IL-12 and NO. In vivo, we found HSP90 expression was elevated in MRL/lpr kidneys when compared to C57BL/6 mice and MRL/lpr mice treated with HSP90 inhibitor 17-DMAG. MRL/lpr mice treated with 17-DMAG showed decreased proteinuria and reduced serum anti-dsDNA antibody production. Glomerulonephritis and glomerular IgG and C3 were not significantly affected by administration of 17-DMAG in MRL/lpr. 17-DMAG increased CD8⁺ T cells, reduced double-negative T cells, decreased the CD4/CD8 ratio and reduced follicular B cells. These studies suggest that HSP90 may play a role in regulating T-cell differentiation and activation and that HSP90 inhibition may reduce inflammation in lupus.     

Reduced development of CD4−8−B220+ T cells but normal autoantibody production in lpr/lpr mice lacking major histocompatibility complex class I molecules

The lpr gene has recently been shown to encode a functional mutation in the Fas receptor, a molecule involved in transducing apoptotic signals. Mice homozygous for the lpr gene develop an autoimmune syndrome accompanied by massive accumulation of double-negative (DN) CD4^?8^?B220⁺ T cell receptor-α/β⁺ cells. In order to investigate the origin of these DN T cells, we derived lpr/lpr mice lacking major histocompatibility complex (MHC) class I molecules by intercrossing them with β2-microglobulin (β2m)-deficient mice. Interestingly, these lprβ2m–/– mice develop 13-fold fewer DN T cells in lymph nodes as compared to lpr/lpr wild-type (lprWT) mice. Analysis of anti-DNA antibodies and rheumatoid factor in serum demonstrates that lprβ2m–/– mice produce comparable levels of autoantibodies to lprWT mice. Collectively our data indicate that MHC class I molecules control the development of DN T cells but not autoantibody production in Ipr/lpr mice and support the hypothesis that the majority of DN T cells may be derived from cells of the CD8 lineage.     

Abnormal Complement-Mediated Immune Clearance in MRL-lpr/+ Heterozygote Mice: Dose-Dependent Effect of theFasAntigen Mutation

We have previously demonstrated decreased complement-mediated clearance of IgG-opsonized erythrocytes in mice homozygous for thelprmutation of thefasgene (BALB/c-lpr/lpr,C57BL/6-lpr/lpr,and MRL-lpr/lpr). To further test the hypothesis that thelprmutation leads to a series of events resulting in selectively decreased complement-mediated immune clearance,in vivoclearance rate data were obtained from MRL-lpr/+ F₁, F₂, and reciprocal backcross mice. Southern analysis of genomic DNA extracted from F₂and backcross mice was used to correlate the presence of the normalfasgene or thelprmutation with normal or decreased complement-mediated clearance, respectively. Mean clearance rate constants for complement-dependent sequestration and phagocytosis were significantly decreased in the group of F₁, F₂, and backcross mice compared to control BALB/c mice (P< 0.0001). Data correlating clearance rate parameters from F₂and backcross mice with their respective genotype demonstrated a dose effect of thelprmutation on abnormal complement-dependent sequestration and phagocytosis (r> 0.98), with heterozygote mice expressing mean values approximately half of those observed in +/+ homozygotes. These data demonstrate that the presence of thelprmutation of thefasgene is strongly correlated in a dose-dependent manner with abnormal complement-mediated immune clearance. This clearance defect may be one mechanism through which thelprmutation acts as an enhancer of autoimmune disease. Reduced clearance of immune complexes would increase the likelihood of tissue deposition and immune-mediated damage.     

T cells of staphylococcal enterotoxin B-tolerized autoimmune MRL-lpr/lpr mice require co-stimulation through the B7-CD28/CTLA-4 pathway for activation and can be reanergized in vivo by stimulation of the T cell receptor in the absence of this co-stimulatory signal

The CD28/CTLA-4 receptors on T cells interact with the B7 molecule on antigen-presenting cells (APC) to produce a co-stimulatory signal that determines the outcome of activation. The role of this co-stimulatory signal in T cell activation and loss of tolerance in autoimmune MRL-lpr/lpr mice has not been investigated previously. The present study examines the contribution of the CD28/CTLA-4 co-stimulatory pathway to the loss of T cell tolerance in Vβ8 transgenic MRL-lpr/lpr and -+/+ mice in which neonatal tolerance has been induced by the superantigen staphylococcal enterotoxin B (SEB). An artificial APC transfected with the murine B7 gene, and a CTLA-4-Ig fusion protein were used to analyze the significance of the CD28/CTLA-4 pathway in vitro. The CTLA-4-Ig fusion protein was also used to inhibit the pathway in vivo. Our results demonstrate that CD28 and CTLA-4 mRNA was overexpressed in the lymph nodes of lpr/lpr mice (MRL, C57BL/6, C3H and AKR), but not in +/+ mice of the same background strain. Lymph node T cells and thymocytes from SEB neonatally tolerized MRL-lpr/lpr mice that had undergone tolerance loss, proliferated when cultured with SEB and B7⁺ fibroblasts in vitro, but did not proliferate when the SEB was presented in the context of B7^? fibroblasts. This in vitro tolerance loss could be prevented by blocking of B7 signaling by CTLA-4-Ig. This loss of tolerance did not occur in lymph node T cells from thymectomized MRL-lpr/lpr mice. SEB challenge of tolerized MRL-lpr/lpr mice in vivo led to weight loss, increased serum cytokine levels and depletion of Vβ8⁺ T cells. These effects were blocked by blocking of the co-stimulatory pathway by treatment with the CTLA-4-Ig fusion protein prior to and during challenge with SEB. T cells from thymus and lymph nodes of these mice did not proliferate later in response to stimulation in vitro with SEB even in the presence of B7⁺ APC. Nonresponsiveness was not due to deletion of Vβ8⁺ CD28⁺ T cells, as the number of these cells was increased after treatment with SEB and the CTLA-4-Ig fusion protein. These results suggest that the response of autoreactive T cells in the thymus and lymph nodes depends on signaling by B7 in vivo and in vitro and that SEB-reactive T cells can be reanergized in vivo by stimulation of the T cell receptor in the absence of signaling through the CD28/CTLA-4 co-stimulatory pathway.     

Clinical effects of mutations to CD95 (Fas): relevance to autoimmunity?

Conclusion The Fas/FasL system has been recognized as a major player in apoptosis, not only in the immune system but in many other tissues of the organism. It is implicated in various biological processes such as physiological cellular turnover, killing by cytotoxic T lymphocytes, protection of tissular integrity, protection of fetus during pregnancy. Its mode of action seems to be rather simple and can be summarized in the equation: Fas + FasL = death. Many pathological processes in humans implying cell death are directly derived from this equation. We have covered different aspects of these pathological situations in humans starting with the lymphoproliferation caused by a defective Fas-dependent apoptosis pathway and responsible for the development of autoimmunity to the Fas-dependent tissue injury as a consequence of a secondary Fas system defect. The complexity of the Fas system in autoimmunity is best recapitulated in the nonobese diabetic (NOD)^lpr/lpr mouse model [9]. On the one hand the lpr mice develop accelerated autoimmunity as a consequence of their Fas defect, on the other hand the destruction of the pancreas in NOD mice is thought to be mediated in part by the interaction between Fas on -islet cells and FasL on diabetogenic infiltrating T lymphocytes. Moreover, these T cells are responsible for the expression of Fas on cells. The intercross of these two mice leads to the paradoxical situation of an autoimmune-prone mouse (lpr) protected from autoimmune diabetes (NOD). One can imagine that in the near future one will be able to modulate the Fas/FasL system for therapeutic purposes.     

gld/gld mice are unable to express a functional ligand for Fas

Mice homozygous for either the lpr or gld genes develop phenotypically identical autoimmune disorders. The gene responsible for the pathology in lpr/lpr mice encodes the Fas antigen, a protein associated with the induction of programmed cell death. To determine if the defect associated with gld represents a mutation in the ligand for Fas, we have assessed the ability of lymphoid cells from homozygous gld/gld mice to lyse target cells in a Fas-dependent manner. Using an antagonistic antibody to Fas, we demonstrate that activated T cells from normal and lpr mice are capable of inducing Fas-mediated lysis of tumor target cells. In contrast, activated T cells from gld/gld mice fail to induce lysis of tumor targets, although cells from gld mice are able to lyse specific allogeneic targets following mixed lymphocyte culture. In addition, activated T cells from gld/gld homozygous animals are not capable of binding to a Fas.Fc fusion protein at high levels, whereas activated T cells from normal and lpr/lpr animals bind Fas.Fc efficiently. These data indicate that mice homozygous for gld are unable to express a functional ligand for Fas.     

TCF1 deficiency ameliorates autoimmune lymphoproliferative syndrome (ALPS)‐like phenotypes of lpr/lpr mice

Autoimmune lymphoproliferative syndrome (ALPS) is an incurable disease, which is characterized by non‐malignant autoimmune lymphoproliferation. TCF1 is a key effector in the canonical Wnt/β‐catenin pathway, regulating the development, activation and function of T cells. In this study, we aimed to explore the potential role of TCF1 in the development of ALPS‐like phenotypes of lpr/lpr mice. We acquired TCF1^−/−lpr/lpr double mutant mice by crossing TCF1 deficiency mice with lpr/lpr mice. Splenocyte compositions, serum cytokines levels, antidsDNA antibody production and kidney pathology were examined in the TCF1^−/−lpr/lpr mice. With these examinations, we revealed that TCF1 deficiency relieved most manifestations of ALPS‐like phenotype, which were caused by Fas mutation in TCF1^−/−lpr/lpr mice. Splenocyte total numbers and compositions were downregulated to the similar levels with wildtype mice. T_E and T_EM cells were decreased in TCF1^−/−lpr/lpr compared with lpr/lpr mice. The levels of autoantibodies and proinflammatory factors in serum, and the histopathology changes and the relative mRNA levels of proinflammatory factors in kidney all displayed parallel tendency in TCF1^−/−lpr/lpr mice. Our study demonstrated that TCF1 deficiency ameliorated the ALPS‐like phenotypes of TCF1^−/−lpr/lpr mice, which might indicate a potential therapeutic direction for ALPS.     

Self-Reactivity and the Expression of Memory Markers Vary Independently in MRL-Mp+/+ and MRL-Mp-lpr/lpr Mice

MRL-Mp-lpr/lpr mice contain phenotypically abnormal populations of T cells, and exhibit an SLE-like autoimmune disease in which autoantibodies are a prominent feature. We analyzed the phenotype and T-cell receptor Vß expression pattern in CD4⁺ T cells of this mutant mouse strain to detect abnormalities that could explain the autoimmunity. The CD4⁺ T cells contain two distinct abnormal populations. One of these expresses B220 and HSA, and in these and other respects closely resembles the accumulating CD4^–CD8^– population. The other expresses a high level of CD44 (Pgp-1), and a high level of the 16A epitope of CD45, and so resembles post-activation T cells. Both of these cell types are exclusive to MRL-Mp-lpr/lpr. We also identified V ß5- and V ß11-positive CD4+ T cells, in both MRL-Mp-lpr/lpr and MRL-Mp-+/+ mice. We conclude that autoimmune T cells can be detected in these mice, but that they are not the cause of the accumulation of abnormal CD4⁺ and CD4^–CD8^–cells.     

Existence of a small population of IL-2Rβhi TCRint cells in SCG and MRL-lpr/lpr mice which produce normal Fas mRNA and Fas molecules from the lpr gene

Mice carrying the lpr gene, SCG and MRL-lpr/lpr mice, were used to characterize the phenotype and lpr gene of abnormally proliferating T cells in these mice. A major population which expanded in these mice were T cells expressing intermediate (int) levels of T cell receptor (TCR) (and CD3) and the phenotype of interleukin-2 receptor (IL-2R)β^lo α^? (possibly abnormal TCR^int cells). The levels of TCR^hi cells of thymic origin (generated through the mainstream of T cell differentiation in the thymus) profoundly decreased after the onset of disease. However, a small population of normal TCR^int cells (i.e. IL-2Rβ^hi α^?) were also found to exist in all tested organs. For example, the majority of abnormal IL-2Rβ^lo TCR^int cells were CD4^?8^? CD2^?, while normal IL-2Rβ^hi TCR^int cells were a mixture of single-positive cells (mainly CD8⁺), CD4^?8^? cells and CD2⁺ cells. Moreover, normal TCR^int cells preferentially produced normal Fas mRNA and Fas molecules from the lpr gene. This phenomenon explains the leaky appearance of normal Fas mRNA and Fas molecules in mice carrying the lpr gene. It is suggested that a small population of IL-2Rβ^hiTCR^int cells are resistant to the lpr genetic abnormality.     

Fas receptor expression on B-lineage cells

Mice homozygous for the lpr mutation have B and T cell defects and develop autoantibodies, suggesting that lpr plays a role in their genesis. The lpr defect has been identified as a mutation in the apoptosis-associated Fas receptor (FasR) gene. To begin to define the role of FasR in B cells, we have surveyed FasR expression on B-lineage cells from early progenitors in the bone marrow through their maturation in the periphery. Contrary to some reports, we found that FasR is expressed on B cells at all stages of their development and is highest on germinal center B cells. FasR is not expressed on lpr/lpr-derived cells. These data are consistent with the idea that lpr/lpr mice have an intrinsic B cell defect that may be manifested in developing as well as peripheral B cells. An unexpected finding is that B-1 (CD5) B cells do not constitutively express FasR: FasR becomes detectable on B-1 B cells only after activation.     

Lack of association of Vβ8+ T cells with lupus-like syndrome in MRL-lpr/lpr mice

To evaluate the role of Vβ8⁺ T cells in the development of lupus-like autoimmune syndrome in MRL-lpr/lpr mice, we treated them with the F23.1 anti-Vβ8 monoclonal antibody (mAb) from birth to 4 months of age. Here we report that almost complete depletion of Vβ8⁺ T cells by the F23.1 mAb treatment neither inhibited nor delayed the development of hypergammaglobulinemia, autoantibody production and autoimmune glomerulonephritis in MRL-lpr/lpr mice. In addition, the F23.1 mAb treatment did not prevent the development of lymphadenopathy and the generation of a CD4^?CD8^? double-negative T cell subset, characteristically accumulating in lpr lymph nodes. Our results strongly argue against the idea that the Vβ8⁺ T cells play a critical role in the development of lupus-like autoimmune syndrome in MRL-lpr/lpr mice.     

Fas-FasL Interaction Involved in Pathogenesis of Ocular Toxoplasmosis in Mice

Ocular toxoplasmosis is a potentially blinding intraocular inflammation. The intent of this study was to investigate the role of Fas-FasL interaction in a murine model of acquired ocular toxoplasmosis induced by intracameral inoculation of Toxoplasma gondii. Intraocular inflammation, Fas and FasL expression on lymphocytes and on ocular tissues, the occurrence of apoptosis, and the frequency of CD8⁺ and CD4⁺ T cells in the infected eyes were analyzed in C57BL/6 (B6) mice. Susceptibility to parasite-induced intraocular inflammation was observed in Fas-deficient (B6-lpr) and FasL-deficient (B6-gld) mice. Inoculation of 5,000 T. gondii tachyzoites induced significant intraocular inflammation associated with increase of Fas and FasL expression in the inoculated eyes of wild-type B6 mice. Flow cytometry demonstrated a significant increase of Fas and FasL expression on the splenocytes from naive mice incubated in vitro with the parasite and on the splenocytes harvested from the infected mice at day 8 after parasite inoculation. Apoptosis of inflammatory cells and cells in ocular tissues was seen, and a greater frequency of CD8⁺ than CD4⁺ T cells was observed in the infected eyes. The intensity of intraocular inflammation was greater in B6-lpr and B6-gld mice than in wild-type B6 mice (P < 0.05). The results suggest that Fas-FasL interaction associated with apoptosis is involved in the pathogenesis of acquired ocular toxoplasmosis in mice.