Biochemical aspects of constitutive and facultative expression of MHC class II molecules at the pre- and posttranslational level.

Biosynthetic properties of Ia antigens derived from B cells, from T cell clone BK-BI-2.6.C6 and from rIFN-gamma induced bone marrow macrophages (BMMO) were evaluated before and after translation. Gross features of B and T cell Ia were found to be very similar. At the pretranslational level no obvious differences were noticed. Following translation in both cases alpha, beta heterodimers associate noncovalently with invariant proteins of the gamma-chain group. These proteins, in particular the principal gamma-chain (p32) become the target of extensive posttranslational modification by sialic acid. The effect is much more pronounced in T cells than in B cells. Unlike in B cells, at the cell surface the bulk of terminally glycosylated gamma-chain (p35) remains attached to the alpha, beta isotypic complexes. This finding renders such T cells very useful for studies on the possible role of the invariant chain during antigen presentation. In BMMO rIFN-gamma elicited high levels of transcripts of polymorphic alpha, beta and invariant gamma chains. However, translation of the polymorphic subunits and their expression at the cell surface occurred in substantial amounts in the total absence of invariant chains. This finding argues against the notion of gamma being required for assembly of alpha, beta isotypes and their intracellular transport to and expression at the plasma membrane.