MAJOR HISTOCOMPATIBILITY COMPLEX CLASS-I EXPRESSION ON NEURONS IN SUBACUTE SCLEROSING PANENCEPHALITIS AND EXPERIMENTAL SUBACUTE MEASLES ENCEPHALITIS

Lack of major histocompatibility class I antigens on neurons has been implicated as a possible mechanism for viral persistence in the brain since these antigens are required for cytotoxic T-lymphocyte recogniti on of infected cells. In subacute sclerosing panencephalitis (SSPE), m easles virus (MV) persists in neurons, resulting in a fatal chronic in fection. MHC class I mRNA expression was examined in formalin-fixed br ain tissue from 6 SSPE patients by in situ hybridization. In addition MHC class I protein expression in MV-infected neurons was examined in experimental Subacute Measles Encephalitis (SME) by double immunohisto chemistry. MHC class I mRNA expression was found to be upregulated in SSPE tissues studied, and in 5 out of 6 cases the expression was defin itively seen on neurons. The percentage of neurons expressing MHC clas s I mRNA ranged between 20 to 84% in infected areas. There was no corr elation between the degree of infection and expression of MHC class I molecules on neurons. Importantly, the number of neurons co-expressing MHC class I and MV antigens was markedly low, varying between 2 to 8% . Similar results were obtained in SME where 20 to 30% of the neurons expressed MHC class I but < 8% co-expressed MHC class I and MV antigen s. Perivascular infiltrating cells in the infected regions in SME expr essed IFN gamma immunoreactivity. The results suggest that MV may not be directly involved in the induction of MHC class I on neurons and th at cytokines such as IFN gamma may play an important role. Furthermore , the paucity of neurons co-expressing MHC class I and MV antigens in SSPE and SME suggests that such cells are either rapidly cleared by cy totoxic T lymphocytes (CTL), or, alternatively, lack of co-expression of MHC class I on MV infected neurons favors MV persistence in these c ells by escaping CTL recognition.