DAMAGE IN B2m GENES AND DNA METHYLATION OF H-2 GENES ARE INVOLVED IN LOSS OF EXPRESSION OF CLASS I MHC PRODUCTS ON THE MEMBRANE OF LR.4, A CELL LINE DERIVATIVE OF THE T-CELL LYMPHOMA L5178Y

We have isolated an H-2 deficient cell line (LR.4) from the T-cell lymphoma L5178Y which grew without restrictions in the peritoneal cavity of different inbred strains of mice. The use of polyclonal anti-H-2 antiserum and complement indicated that LR.4 cells did not express class I determinants on the cell membrane. Southern blots of genomic DNA of LR.4 cells showed that B2m genes were severely damaged and that class I H-2 genes were extensively methylated. Consequently, LR.4 cells failed to transcribe mRNAs for both B2m and class I H-2 genes. On the other hand, specific immunity to LR.4 was demonstrated in C57BL/6J mice since, in subsequent challenges with either LR.4 or EL4.4, LR.4 did not grow, whereas EL4.4 grew and killed the mice. In C57BL/6J mice, rejection of LR.4 was accompanied by the production of cytotoxic antibodies. The immune response induced in C57BL/6J mice was determined by non-H-2 antigenic determinants in LR.4 cells.     

Use of V beta.8 genes in splenic Lyt-2+ cytotoxic lymphocyte precursors reactive to bm1 or bm14 alloantigen in individual C57BL/6 mice

The cytotoxic response of cell sorter-purified small Lyt-2+ splenic cytotoxic lymphocyte precursors from 10 individual C57BL/6 mice to mutant class I H-2Kbm1 or H-2Dbm14 allodeterminants was analyzed under limiting dilution conditions. The cytotoxic activity of anti-bm1-specific or anti-bm14-specific cytotoxic T lymphocyte (CTL) populations (selected for a high probability of clonality) was tested against F23 hybridoma cells; F23+ CTL clones lysed F23 hybridoma targets but F23- CTL clones did not. In the C57BL/6 anti-bm1 mixed lymphocyte reaction, 36% (range 29-48%) of the generated CTL clones were F23+; in the B6-anti-bm14 mixed lymphocyte reaction, 45% (range 34-49%) of the generated CTL clones were F23+. Hence, a large fraction of the anti-bm1- or anti-bm14-reactive CTL clones from C57BL/6 mice use V beta.8 genes to construct these allospecific T cell receptor phenotypes, but no extensive variation in the use of V beta.8 genes in the construction of allospecific T cell receptor phenotypes of restricted heterogeneity is found in individual mice of the same strain.     

Role of H-2 and non-H-2 genes in control of bacterial clearance from the spleen in Salmonella typhimurium-infected mice.

The ability of mice to clear Salmonella typhimurium from their spleens in the late phase of infection was studied after inoculation with a temperature-sensitive mutant. Clearance of bacteria was delayed in C57BL/6 mice compared with BALB/c, C3H/HeJ, DBA/2, A/J, and CBA mice. The responses of F1 hybrids, backcrosses, and recombinant inbred strains derived from C57BL/6 and BALB/c (both Itys) and of H-2 congenic mice were analyzed. The results showed that the low rate of bacterial clearance was recessive, that the rate of clearance was under polygenic control, and that an H-2-linked gene(s) plays a major role. Among H-2 congenic mice with a C57BL/10 background, three phenotypes of bacterial clearance could be distinguished: high (H-2j, H-2q, and H-2u), intermediate (H-2d, H-2f, H-2k, H-2p, H-2r, H-2s, and H-2v), and low (H-2b) rates. The effect of the H-2 complex was apparent with different genetic backgrounds (Itys and Ityr). In recombinant inbred strains derived from C57BL/6 (Itys) and A/J (Ityr) mice, the effect of the H-2b haplotype on bacterial clearance appeared to be fully expressed only in strains carrying the Itys allele.     

Identification of murine H-2Db histocompatibility antigens in cells transfected with cloned H-2 genes

Clones of mouse L-cells transformed with 21 cosmids containing 15 major histocompatibility complex class I genes of C57BL10 (H-2b) sperm cell DNA were analyzed for the expression of their transfected H-2 and Qa/Tla genes. Three cosmids contained a single gene, mapping to the H-2D region. This gene encodes the H-2Db alloantigen: mouse L-cells transfected with cosmids containing this gene reacted with monoclonal antibodies and alloantisera specific for the H-2Db antigen and expressed a 46-kd H-2 heavy chain associated with beta 2-microglobulin in their cell membranes. Furthermore, these transfected cells were stimulators of, and targets for, anti-H-2Db cytotoxic T-lymphocytes. Eighteen cosmids contained 14 different genes mapping to the Qa and Tla regions. L-cells transfected with these genes did not express class I genes reacting with alloantisera or monoclonal antibodies against Qa2, Qa4 or TL differentiation antigens. In particular, the Qa2,3 gene of C57BL10 was not identified.     

Lethal effect of the intravenous injection of H-2 alloantigen activated T cells.

T cells of C57BL/6J (B6) mice activated in vitro against the K and D determinants of the H-2d haplotype, and expanded in tissue culture in the presence of interleukin 2, were capable of rapidly killing lethally irradiated BALB/c recipients when injected intravenously. Mice given 10(6), 10(7) and 10(8) such T cells survived 10.5, 8.5 and 0.5 days respectively. Mice given irradiation only survived 10.5 days. Mice given 10(8) B6 T cells activated against all determinants of a third party haplotype (H-2k) survived 8.5 days. Respiratory distress developed within 3 hr of injection in mice given 10(8) T cells activated against the H-2 alloantigens of the recipient haplotype, and at autopsy their lungs showed pulmonary congestion, focal obliteration of the alveolar space and thickening of alveolar interstitial tissue with fibrin, red cell and a mononuclear cell infiltrate. These findings may have relevance to the use of cellular immunotherapy.     

Specific recognition and rejection of the H-2-deficient cell line LR.4 By C57BL/6J mice

The humoral immune response developed by C57BL/6J mice against the β₂-microglobulin (β₂m) and major histocompatibility complex (MHC) class I- and class II-deficient cell variant of L5178Y, LR.4, is strain specific, is not linked to a given haplotype and involves at least one antigenic determinant expressed on the cell membrane. Anti-LR.4 antibodies can be detected in the serum and ascitic fluid of tumour-bearing animals, and in the serum of mice immunized with mitomycin C (MC)-treated cells. In vitro, cytotoxic T lymphocytes (CTL) cannot be induced under different experimental conditions. However, recognition and lysis of LR.4 axe mediated by an antibody-dependent cell-mediated cytotoxicity (ADCC) mechanism in which natural killer (NK) cells extracted from the spleen of resistant or susceptible strains are the effector cells. The NK cells responsible for ADCC against LR.4 are not inducible with polyinosinic-polycytidylic acid (poly(I:C)) and could represent a subset that is not detectable by conventional assays. In conclusion, the incapacity of BALB/c and possibly of other strains of mice to reject LR.4 is determined by the failure to mount a humoral immune response.     

Up-regulation of MHC class I expression accompanies but is not required for spontaneous myopathy in dysferlin-deficient SJL/J mice

We found that up-regulation of major histocompatibility complex (MHC) class I expression accompanies, but is not required for, appearance of spontaneous myopathy in SJL/J mice. In some neuromuscular diseases, MHC class I expression is markedly up-regulated in muscles, though the consequences of this up-regulation for pathology are not clear. To study MHC class I in myopathy, we compared muscles of SJL/J mice to muscles of SJL/J mice that were also MHC class I-deficient due to targeted mutation in the beta-2-microglobulin gene (SJL/J B2m (-/-) mice). SJL/J mice show spontaneous myopathy and have a mutation in the dysferlin gene, a gene which is also mutated in human limb-girdle muscular dystrophy type 2B (LGMD2B). Muscles of eight-month-old SJL/J mice had higher levels of MHC class I expression than muscles of either C57BL/6J (wild-type) or SJL/J B2m (-/-) mice. In contrast, the percentage of abnormal muscle fibers was similar in SJL/J and SJL/J B2m (-/-) muscles. Invading Mac-1(+) cells were most abundant in SJL/J B2m (-/-) muscles, moderately abundant in SJL/J muscles, and rare in C57BL/6J muscles. Thus, MHC class I was markedly up-regulated in SJL/J muscles, but this high level of MHC class I was not necessary for the appearance of myopathy.     

H-2-linked murine cytotoxic T cell responses specific for sendai virus-infected cells.

CBA (H-2k) mouse-derived lymphochoriomeningitis virus and herpes simplex virus-specific cytotoxic T lymphocytes lyse virus-infected target cells compatible on either the H-2k or H-2D region. In contrast, CBA, C3H and AKR (H-2k) mouse-derived sendai virus-specific cytotoxic T lymphocytes (CTL) fail to lyse H-2D-compatible virus-infected cells. A similar lack of H-2D region-associated lytic activity was found with C57BL/6 and C57BL/10 (H-2b) mice as well as with the recombinants B10.A (2R) [Kb-Db] and B10.A (4R) [Kk-Db]. On the other hand, BALB/c (H-2d) mice and A/J (H-2a) mice do generate H-2Dd-associated sendai virus-specific CTL. These results are in contrast to those obtained with (CBA X BALB/c)F1 and B10.HTT [Ks-Dd] mice, which failed to mount Dd region-associated CTL responses. It is concluded that D region-associated sendai virus-specific CTL responsiveness varies with the H-2 genotype of the responder cells.     

Searching for MHC-restricted anti-viral antibodies: antibodies recognizing the nucleoprotein of influenza virus dominate the serological response of C57BL/6 mice to syngeneic influenza-infected cells

An attempt has been made to generate monoclonal antibodies which recognize the same target structures on influenza-infected cells as those seen by cytotoxic T lymphocyte (CTL) receptors. Such antibodies, if they mimicked the T cell receptor specificity, would be expected to be both virus specific and restricted in their binding by the major histocompatibility complex (MHC) antigens. Approximately 200 hybridomas from C57BL/6 (H-2b) mice primed and boosted with influenza virus (X-31)-infected EL4 (a C57BL/6 T cell lymphoma) were screened for reactivity on infected and uninfected cells of different MHC haplotypes. Of the 10 hybridoma antibodies which were identified as being reactive with X-31-infected EL4, but not uninfected EL4, all reacted equally well with X-31-infected cells of H-2b, H-2d and H-2k haplotypes, indicating a lack of MHC restriction in their recognition of the infected cells. Unexpectedly, 7 of the 10 monoclonal antibodies were found to react specifically with the purified influenza virus nucleoprotein (NP), a predominant viral antigen in CTL recognition of infected cells. Fluorescence-activated flow cytometry confirmed that these antibodies were able to recognize NP serological determinants on the surface of viable, infected cells, but the anti-NP antibodies were unable to block the lytic activity of an NP-specific CTL clone.     

Immune response gene regulation of the humoral immune response to Porphyromonas gingivalis fimbriae in mice.

Among various strains of mice immunized orally with Porphyromonas gingivalis fimbriae and adjuvant GM-53 in liposomes, BALB/c and DBA/2 mice (H-2d) were found to be high responders to the fimbriae, CBA/J and C3H mice (H-2k) were intermediate, while C57BL/6 mice were low responders in terms of serum IgG and salivary IgA responses. Furthermore, humoral immune responses were examined using congeneic mice of B10 background showing different H-2 haplotypes, and it was revealed that B10.D2 mice (H-2d), followed by B10.BR (H-2k), responded well to antigenic stimulation of the fimbriae, while C57BL/10 mice (B10, H-2b) were low responders to the fimbriae. Hybrids between BALB/c and C57BL/6 mice were found to reflect a phenotype of low responders. Thus, the humoral immune responses to P. gingivalis in mice are restricted by H-2 haplotype.     

Augmentation of the antibody response by hapten help. II. Determination by many genes outside the MHC

The purpose of the present work was to characterize the immune response (Ir) genes that influence augmentation of the antibody response by help with the hapten azobenzene-arsonate (ABA). Hapten help was measured as the augmentation in the plaque-forming cell (PFC) response to bovine gamma globulin (BGG) after priming mice with ABA conjugated to ovalbumin (OVA) and challenging subsequently with ABA-BGG. The first approach involved inbred mouse strains that were matched for H-2 but differed in their non-H-2 genetic backgrounds. B10.D2 mice were low responders even though they shared the H-2d haplotype with BALB/c and DBA/2 mice, which were high responders. C57BL/10 mice were high responders even though they shared the H-2b haplotype with C57BL/6, C3H.SW, and A.BY mice, all of which were strains that were low responders. The second approach was to identify any segregation of H-2 with the gene(s) encoding susceptibility to hapten help in the backcross generation. (BALB/c X C57BL/6) F1 hybrids were backcrossed to C57BL/6. The results showed no association with the major histocompatibility complex (MHC).     

Inhibition of VLA-4 and up-regulation of TIMP-1 expression in B16BL6 melanoma cells transfected with MHC class I genes

The effect of MHC class I gene transfection on the metastatic properties of B16BL6 melanoma cells was investigated. BL6-8 melanoma cells transfected with H-2K or H-2K, but not H-2D or H-2L, genes showed a dramatic reduction in their ability to generate experimental metastases in immunosuppressed CB6F1 mice. This observation suggested that some changes in the metastatic phenotype may have been induced in the H-2K- transfected melanoma cells. Analyses of adhesive and invasive properties of BL6-8 melanoma cells transfected with H-2 class I genes have been performed. We found that the loss of metastatic properties in the H-2K or H-2K gene-transfected melanoma cells was associated with reduced adherence to endothe-lial cells, laminin and collagen IV, decreased ability to form homotypic cell aggregates and with a complete loss of VLA-4 integrin expression. In addition, BL6-8 melanoma cells transfected with H-2K genes demon-strated reduced ability to invade Matrigel that paralleled up -regulation of TIMP-1 expression. Incubation of untransfected BL6-8 clone or B16F1 cells with 5-azacytidine similarly resulted in up-regulation of TIMP-1, suggesting that the changes in methylation of TIMP-1 gene could be responsible for TIMP-1 expression in the H-2K-transfected BL6-8 melanoma cells. Transfection of BL6-8 cells with the H-2D d /L d genes did not affect their adhesive and invasive properties. Previously we reported that reduction in the metastatic properties of the H-2K b transfected cells was associated with alterations in cell surface carbohydrates with appearance of a-galactosyl epitopes and reduction in cell surface sialylation. The present data indicate that, in addition to changes in cell surface carbohydrates, reduction in adhesive properties and up-regulation of TIMP-1 may be responsible for the observed loss of metastatic potential of BL6-8 cells transfected with the H-2K genes. © Rapid Science Ltd.     

Lack of F1 anti-parental resistance in H-2b/d F1 hybrids devoid of β2-microglobulin

F₁ hybrid mice often reject parental hematopoietic grafts, a phenomenon known as hybrid resistance. Hybrid resistance is mediated by natural killer (NK) cells and although the molecular interactions responsible for this phenomenon are largely unknown, one hypothesis suggests that parental cells are rejected because they fail to express a complete set of host major histocompatibility complex (MHC) class I molecules. Inherent in this theory is that NK cells in the F₁ hybrid are instructed by self MHC class I molecules to form an NK cell repertoire capable of reacting against cells lacking these self MHC class I molecules. Here, we show that C57BL/6 x DBA/2 mice (H-2^b/d) devoid of β₂-microglobulin (β₂m) are incapable of rejecting β₂m?/? parental C57BL/6 cells (H-2^b) both in vivo and in vitro. From this, we conclude that the development of an NK cell repertoire, at least in F₁ mice of the H-2^b/d haplotype, requires expression of MHC class I molecules complexed with β₂m.     

Different Expressions of Ly-49 Receptors on Mouse NK and NK T Cells

NK T cells are a unique T cell lineage and are reported to express Ly-49 molecules which are inhibitory receptors specific for class I molecules. In this study, we examined the expression of activation and inhibitory receptors on NK T cells in different organs of beta2-microgloblin knock out (beta2mKO), C57BL/6 (B6; H-2b), C57BL/10 (B10; H-2b) and B10.D2 (H-2d) mice. The low level expression of inhibitory receptors Ly-49A and G2 on NKT cells as well as NK cells, which are specific for Dd antigen, were observed in B10.D2 mice, but not in beta2mKO, B6, or B10 mice. The small percentage of inhibitory receptor Ly-49C positive NK and NKT cells, which is specific for Kb and Dd antigens, was observed in BMC, LMC and SC of B6, B10 and B10.D2 mice compared to beta2mKO mice. On the contrary, the large percentage of Ly-49C positive NK T cells was observed in thymocytes of B6, B10 and B10.D2 mice compared to beta2mKO mice. Interestingly, Ly-49D activation receptor was hardly detectable on NK T cells in any organs of the 4 strains of mice whereas it was clearly detectable on NK cells. These findings suggest that the unique characteristics of NK T cells may mediate regulatory function in MHC class I antigen-restricted immunity.     

Southern blot analysis of major histocompatibility genes of lpr mice

Several recent functional and immunofluorescent studies have suggested that abnormal major histocompatibility genes may be expressed in mice homozygous for the autoimmune disease-accelerating lpr (lymphoproliferation) gene. In an effort to establish the molecular basis of the expression of abnormal MHC molecules in these mice, we used MHC class I- and class II-specific cDNAs to probe endonuclease-digested genomic DNA from strains expressing lpr to look for restriction fragment length polymorphisms (RFLPs). The RFLP pattern of MRL/Mp-lpr/lpr (MRL/lpr) DNA, as determined using five restriction enzymes, was identical to that of MRL/MP-+/+ and typical of the H-2k haplotype exhibited by C3H/HeSnJ and AKR/J, which are the two H-2k haplotype strains from which MRL was derived. DNA from C57BL/6-lpr/lpr (B6/lpr) exhibited a pattern similar to C57BL/6, again indicating that the lpr gene did not alter MHC genes, within the limitations of this approach. Because macrophages derived from lpr mice had been specifically reported to express altered MHC haplotypes, DNA was prepared from MRL/lpr peritoneal macrophages and Southern blotting was performed using probes for I-A beta and I-E beta. The patterns observed were typical of H-2k DNA. The data thus indicate that the abnormalities of MHC genes previously observed in lpr mice may be due to the influence of other genes on MHC gene products, but are probably not due to alterations in MHC genes themselves.     

Polygenic influences on the length of oestrous cycles in inbred mice involve MHC alleles.

Genetic influences on female reproductive cycles were analysed in histocompatibility-congenic strains of mice. Oestrous cycles of young, virgin mice of inbred-congenic strains, hybrid crosses (F1), and parental-hybrid backcrosses (F2) were monitored for 3 months. Oestrous cycles were categorized by length (inter-oestrous interval): 4, 5, 6, or 7-14 days. Mice with the following H-2 haplotypes had a greater proportion of 5-day oestrous cycles: H-2b, H-2r, H-2h2, H-2h4, and H-2i5. In contrast, the H-2k and H-2d haplotypes had mostly 4-day oestrous cycles. Influences of H-2 haplotype were seen on two genetic backgrounds, C57BL/10Sn and C3H. Non-H-2 alleles were also implied by different patterns of cycles between strains with the same H-2b haplotype: C57BL/10Sn with predominantly 5-day cycles vs. C57BL/6J with a mix of 4- and 5-day cycles. The genetic basis for strain differences was investigation in F1 hybrids and their backcrosses. F1 hybrids of an H-2b (C57BL/10Sn; 5-day cycles) and an H-2k (B10.BR; 4-day cycles) strain had mostly 5-day cycles, indicating dominance of an H-2b allele(s). However, F1 hybrids from the reciprocal B6 x B10 cross (both H-2b) also display a preponderance of 5-day cycles, indicating dominance of a non-H-2 autosomal allele from the C57BL/10Sn strain. Among F2 mice, a '4-day' phenotype segregated with homozygosity for the k haplotype (P < 0.05, chi 2). These findings demonstrate the influence of genetic differences at the major histocompatibility complex on oestrous cycles.     

The Availability for Recognition of Normal H-2 Antigens by Cytotoxic T Lymphocytes on a Rauscher-Virus-transformed Cell, RBL-5A

The availability for recognition of cell surface H-2Db antigenic determinants on RBL-5A cells in comparison to normal C57BL/L cells was investigated by an in vitro immunological assay. No differences in the immunological recognition of the H-2Db antigens on RBL-5A cells compared to normal C57BL/L cells were detected. This assay system, however, readily detected changes in the H-2Kb determinant profile on target cells from the C57BL/L H-2Kb region mutant strains H(zl) and H(zl70) when compared to normal C57BL/6 mice. The result obtained with RBL-5A target cells therefore suggests that Rauscher murine leukaemia virus transformation does not induce, either genotypically or phenotypically, an immunologically recognizable alteration of the H-2Db antigen profile on this cell.     

Spleen cell cytokine secretion in Mycobacterium bovis BCG-infected mice.

Three susceptible mouse strains, i.e., BALB/c (H-2d), C57BL/6 (H-2b), and major histocompatibility complex-congenic BALB.B10 (H-2b), were infected intravenously with 4 x 10(6) CFU of live Mycobacterium bovis BCG and analyzed 4 weeks later for in vitro spleen cell cytokine secretion in response to purified protein derivative (PPD), BCG culture filtrate (CF), BCG cellular extract, total BCG, the purified extracellular 30-32-kDa antigen (the fibronectin-binding antigen 85), or the intracellular 65-kDa heat shock protein. C57BL/6 and BALB.B10 mice produced 5- to 10-fold more gamma interferon and interleukin-2 (IL-2) when stimulated with CF, PPD, and antigen 85 than BALB/c mice did. When stimulated with BCG extract and whole BCG, gamma interferon and IL-2 levels were generally lower and comparable in the three strains. IL-4 was detected in spleen cell culture supernatants from infected BALB/c mice but not from C57BL/6 or BALB.B10 mice. IL-5 could not be detected. C57BL/6 and BALB.B10 spleen cells also produced more tumor necrosis factor alpha and IL-6 after stimulation with PPD and CF than BALB/c cells did. Finally, BCG vaccination generated efficient protective immunity in C57BL/6 and BALB.B10 mice but not in BALB/c mice. These data suggest that secreted mycobacterial CF antigens selectively induce a strong TH1 response in BCG-infected C57BL/6 and BALB.B10 mice, whereas in BALB/c mice this response is partly counterbalanced by TH2 cells.     

Correlation between levels of immunoglobulins and immune complexes in plasma of C57BL/6 and C57L/J mice infected with MAIDS retrovirus.

Infection with helper-free, defective MAIDS murine leukemia virus (MuLV) caused a rapid polyclonal activation of B cells in 0.75-, 2-, and 6-month-old C57L/J mice (H-2b, Fv-1n/n), similar to that in C57BL/6 mice (H-2b, Fv-1b/b), which was recognized by elevated plasma immunoglobulin concentrations. However, changes in plasma immunoglobulin levels differed in C57BL/J and C57BL/6 mice. In C57L/J mice, infection resulted in a rapid increase in plasma IgM and IgG2a, and the elevation of IgG2a persisted undiminished for 21 weeks. Levels of IgG2b also became slightly elevated, but those of IgG1 and IgG3 were not significantly affected. Plasma of 6 to 7-month-old C57BL/6 mice contained already high levels of IgM (30-40 mg/ml), which persisted undiminished in uninfected mice but decreased progressively in infected mice to 10% of the original concentration during 25 weeks of observation. In C57BL/6 mice, plasma IgG1 and IgG2b as well as IgG2a became similarly elevated after infection but also only transiently. Their levels began to decrease progressively about 10 weeks after infection and fell to far below the maximum concentration observed. The drastic loss of plasma IgM and IgGs observed in C57BL/6 mice during the later stages of MAIDS MuLV infection did not seem to be a consequence of the polyclonal activation of B cells per se but seemed to reflect additional immunological abnormalities arising in infected C57BL/6 but not C57L/J mice. In both mouse strains these changes in plasma Ig levels correlated with the formation of Ig-containing immune complexes that bound to high-affinity, protein-binding ELISA plates in the absence of antigen coating, which may represent unusual forms of self-antigen-antibody complexes.     

A gene(s) within the H-2D region determines the development of toxoplasmic encephalitis in mice.

Studies were performed in a murine model to determine if there is genetic control of the development of toxoplasmic encephalitis. Ten weeks after infection with the ME49 strain of Toxoplasma gondii, mice with the H-2b haplotype (C57BL/6, C57BL/10) and H-2k haplotype (C3H/He, CBA/J) developed remarkable inflammatory changes in their brains, whereas mice with the H-2a haplotype (A/J) and H-2d haplotype (BALB/c, DBA/2) did not. In the area of acute focal inflammation in mice with the H-2b and H-2k haplotypes, tachyzoites and toxoplasma antigens were demonstrated by immunoperoxidase staining, suggesting that the focal inflammation was induced by toxoplasma organisms. B10 congenic mice were used for further analysis of this genetic regulation. Presence of the encephalitis in B10 and B10.BR but not in B10.A and B10.D2 mice at 10 weeks after infection indicated regulation of the inflammation by a gene(s) within the H-2 complex. The encephalitis developed in B10.A (2R) and B10.A (4R) mice but not in B10.A (3R) and B10.A (18R) during infection. These results clearly indicated that the development of toxoplasmic encephalitis was controlled by a gene(s) in the H-2D region. The Qa and Tla genes did not appear to be critical in determining susceptibility to the encephalitis. There was no correlation between serum toxoplasma antibody titres and occurrence of the encephalitis. Injection of a monoclonal antibody to interferon-gamma (IFN-gamma) remarkably augmented the inflammatory changes in the brains of the infected B10 mice. In contrast, the treatment did not induce any inflammatory response in the brains of the infected BALB/c mice. A similar genetic regulation may be operative in determining development of toxoplasmic encephalitis in AIDS and other immunocompromised patients.