I-E transgenic mice: a model system to dissect the regulation and function of MHC class II genes in vivo

Immune responsiveness against allogeneic platelet transfusions is determined by the recipient's major histocompatibility complex class II phenotype

BACKGROUND

Immunoglobulin G (IgG) anti-platelet (PLT) immunity has been shown to be initiated by indirect allorecognition where recipient T cells recognize donor PLT antigens presented by class II molecules encoded by the major histocompatibility complex (MHC) on recipient antigen-presenting cells. To understand how the recipient's MHC class II molecules may influence PLT alloimmunity, immune responsiveness against transfused PLTs was tested in different mouse strains.

STUDY DESIGN AND METHODS

Various inbred and mutant mouse strains were transfused with allogeneic PLTs and IgG donor antibodies were measured by flow cytometry.

RESULTS

When recipient mice, expressing both MHC class II I-A and MHC class II I-E molecules, were transfused weekly with allogeneic PLTs, high titers of IgG donor antibodies were generated. In comparison, however, recipient mice expressing only MHC class II I-A molecules had significantly (p < 0.001) reduced IgG antibody responsiveness against PLT transfusions. The low IgG responder status against allogeneic PLT transfusions was rescued in transgenic mice expressing I-E molecules and in mice genetically deficient in either beta2-microglobulin or CD8+ T cells.

CONCLUSION

IgG immune responsiveness against allogeneic PLT transfusions is dependent on recipient expression of I-E MHC class II molecules, whereas I-A expression is linked with CD8-mediated suppression of PLT immunity. The data suggest that strategies to modify recipient MHC class II presentation of donor PLT antigens would be effective in eliminating PLT alloimmunity.

Survival of Naive CD4 T Cells: Roles of Restricting Versus Selecting MHC Class II and Cytokine Milieu

The diversity of naive CD4 T cells plays an important role in the adaptive immune response by ensuring the capability of responding to novel pathogens. In the past, it has been generally accepted that naive CD4 T cells are intrinsically long-lived; however, there have been studies suggesting some CD4 T cells are short-lived. In this report, we identify two populations of naive CD4 T cells: a long-lived population as well as a short-lived population. In addition, we identify two factors that contribute to the establishment of long-lived naive CD4 T cells. We confirm earlier findings that MHC class II interaction with the TCR on CD4 T cells is important for survival. Furthermore, we find that MHC class II alleles with the correct restriction element for Ag presentation mediate the peripheral survival of naive CD4 T cells more efficiently than other positively selecting alleles, regardless of the selecting MHC in the thymus. The second component contributing to the survival of naive CD4 T cells is contact with the cytokines IL-4 and IL-7. We find that the physiological levels of IL-4 and IL-7 serve to enhance the MHC class II-mediated survival of naive CD4 T cells in vivo.

Mice lacking the MHC class II transactivator (CIITA) show tissue-specific impairment of MHC class II expression

CIITA activates the expression of multiple genes involved in antigen presentation and it is believed to be required for both constitutive and IFN gamma-inducible expression of these genes. To understand the role of CIITA in vivo, we have used gene targeting to generate mice that lack CIITA. CIITA-deficient (-/-) mice do not express conventional MHC class II molecules on the surface of splenic B cells and dendritic cells. In addition, macrophages resident in the peritoneal cavity do not express MHC class II molecules upon IFN gamma stimulation nor do somatic tissues of mice injected with IFN gamma, in contrast with wild-type mice. The levels of Ii and H-2M gene transcripts are substantially decreased but absent in CIITA (-/-) mice. The transcription of nonconventional MHC class II genes is, however not affected by CIITA deficiency. A subset of thymic epithelial cells express MHC class II molecules. Nonetheless, very few mature CD4 T cells are present in the periphery of CIITA (-/-) mice despite MHC class II expression in the thymus. Consequently, CIITA(-/-) mice are impaired in T-dependent antigen responses and MHC class II-mediated allogeneic responses.

Intrathymic and extrathymic tolerance in bone marrow chimeras

Parent-->F1 bone marrow (BM) chimeras provide a useful model for studying self tolerance induction. When prepared with supralethal irradiation (1300 cGy) and conditioned with anti-T cell antibodies, parent-->F1 BM chimeras are devoid of host BM-derived cells; host H-2 expression is apparent in both the intrathymic and extrathymic environments but is limited to non BM-derived cells. When parent-->F1 chimeras are injected with T cells from normal parental strain mice, the expression of host H-2 antigens on nonprofessional APC might be expected to induce tolerance through induction of clonal anergy. In practice, this does not occur. Instead, a small proportion of the injected T cells is induced to proliferate and differentiate into effector cells. Tolerance is not seen. Similarly, tolerance is not apparent when thymectomized parent-->F1 chimeras are given parental strain thymus grafts. These findings suggest that the expression of host H-2 antigens in the post-thymic environment of chimeras is not intrinsically tolerogenic for mature T cells or recent thymic emigrants. Interestingly, post-thymic tolerance does occur when parental strain T cells differentiate in the endogenous thymus of chimeras. Thus, when mature CD8+ cells are prepared from thymus vs lymph nodes (LN) of parent-->F1 chimeras, tolerance to host class I antigens is more marked in LN than thymus; this applies to cytotoxic T lymphocyte (CTL) precursors, generated by limiting dilution analysis. It would appear therefore that many of the host-reactive CTL precursors generated in the thymus of chimeras undergo tolerance induction (deletion or irreversible inactivation) in the post-thymic environment. We suggest that such tolerance is a reflection of a covert form of tolerance induced in the thymus: intrathymic contact with host antigens on thymic epithelial cells (TEC) in chimeras does not delete typical CTL precursors, but these cells are rendered "semi-tolerant". When cultured in vitro in the presence of lymphokines, the cells are able to recover and differentiate into CTL. In vivo, however, the cells recognize antigen in the periphery in the relative absence of lymphokines and the cells die. Although host class I expression on TEC in chimeras deletes only a small proportion of CTL precursors, contact with TEC induces strong tolerance of CD8+ cells in terms of helper-independent proliferative responses in vitro and induction of lethal graft-versus-host disease in vivo. We postulate that these latter responses are controlled by high-affinity T cells, whereas typical CTL generated in LDA are predominantly low-affinity cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Selection of the T-cell repertoire in transgenic mice expressing a transplantation antigen in distinct thymus subsets

Transgenic mice that expressed a transplantation antigen, H-2Kb, in an unusual tissue distribution have been developed. Gene-regulatory elements from the immunoglobulin heavy-chain locus (Emu enhancer and heavy chain promoter) were linked to the class I Kb gene and the construct microinjected into fertilized mouse eggs of a different haplotype. It was expected that such gene-regulatory elements would direct expression of the foreign class I molecules only to B and T lymphocytes. However, expression was also detected in a subset of thymus medullary epithelium. The Kb molecules expressed on this thymic subset were unable to positively select T cells for passage to the periphery. The mice were, however, tolerant of the cell types expressing the foreign Kb molecules and were also tolerant of Kb presented as skin grafts. These results suggest that not all components of thymic epithelium are involved in positive selection of T cells and that transplantation antigens expressed on non-dendritic cells can induce tolerance.