The regulation of expression of MHC products in mice. Factors determining the level of expression in kidneys of normal mice.

Cytokines, especially IFN, have been shown to increase MHC expression above basal levels in many immunologic processes, but their contribution to normal MHC expression is unknown. Environmental stimuli, such as bacterial LPS, may provoke the secretion of cytokines which could influence MHC expression in the normal host. We therefore studied the factors influencing the normal expression of MHC products in mice, by using kidney as a representative nonlymphoid tissue. Considerable variation in MHC expression was observed between otherwise normal individuals by using indirect immunoperoxidase staining and a radiolabeled antibody binding assay. An intact T cell compartment was not necessary for normal levels of expression; in athymic nude mice and mice with severe combined immunodeficiency disease, MHC expression was similar to or greater than that of controls. Neither a course of broad spectrum antibiotics nor genetically determined resistance to LPS influenced the level of normal expression. Mice raised under germ-free conditions, on a chemically defined diet also had similar levels of MHC expression when compared with mice maintained under conventional conditions. However, when germfree mice were placed in a conventional colony, induction of both class I and II MHC products occurred. Thus, although exposure to bacterial flora or other sources of endotoxin was not required for normal MHC expression, a change in flora can up-regulate expression, probably by inducing the secretion of cytokines from non-T cells. Treatment with cyclosporine or anti-IFN antibodies produced, at most, a small reduction in the level of renal MHC expression. Therefore in some mice, a small component of MHC expression may reflect the secretion of cytokines in response to stimuli such as bacterial LPS. However, most MHC expression in nonlymphoid tissues is constitutive and independent of a physiologic IFN response.