“…Since it was established that MHC antigens formed an important part of the antigenic struc-165 ture recognised by T lymphocytes (Zinkernagel and Doherty, 1979;Schwartz, 1986), there "has been great interest in the regulation of MHC antigen expression on potentJal antigen-presenting cells. It is well documented that different virus infections may either inhibit or enhance the expression of MHC antigens, Notably, infection by adenovirus (F~i~lbo et al, 1986), herpes simplex virus type 2 (Jennings et aL, 1985), ectromeiia virus (Gardner et al, 1975) and measles virus (Rager-Zisman et al, 1981) can down-regulate class I MHC antigen expression of their host cells and may make the latter less susceptible to cytotoxic T cells. On the other hand, murine hepatitis virus (Massa et al, 1986;Suzumura et al, 1986), Epstein-Barr virus (McCune et al, 1975, Moloney murine leukemia virus (Flyer et ai., 1985) and Theiler's murine encephalomyelitis virus (Rodriguez et al, 1987) can up-regulate class I and/or class II MHC antigen expression.…”