INTERPRETING MHC CLASS-I EXPRESSION AND CLASS-I CLASS-II RECIPROCITY IN THE CNS - RECONCILING DIVERGENT FINDINGS

MHC-restricted T cells are thought to contribute to clinical demyelina tion in MS and other circumstances. The step-by-step mechanisms involv ed and ways of controlling them are still being defined. Identificatio n of the MHC(+) cells in the CNS in situ has been controversial. This chapter reviews MKC expression in neural tissue, including normal, pat hological, experimental, and developing tissue in situ and isolated ce lls in vitro. A basic pattern is defined, in which MHC expression is l imited to nonneural cells and strongest class I and II expression are on different cell types. Variations from the basic pattern are reviewe d. Ways of reconciling divergent findings are discussed, including the use of ''mock tissue'' to help choose between technical and biologica l bases for divergent findings, the potential contribution of internal antigen to the in situ staining patterns, and the possibility that cl ass I upregulation is actively suppressed in situ. Functional implicat ions of the observed patterns of MHC expression and ways of confirming the function of each MHC(+) cell type in situ are described. It is su ggested that modulating MHC expression in different cell types at diff erent times or in different directions might be desirable. (C) 1995 Wi ley-Liss, Inc.