Control of MHC Gene Expression
The overall organization of the MHC has become clear during the last few years. The molecular genetic organization of both Class I and Class II genes have revealed unsuspected complexity. The number of Class I sequences ( ~ 30) were a surprise. The number of Iaα and β chains while fewer, (5 α and 7 β genes in man) are still considerable. The central questions of MHC genetics have changed from the number and structure of genes to their regulation. A number of systems have been defined to examine changes in Class II gene expression. Among the best examined so far are the effects of IFγ on Class II antigen expression. Here it is clear that IFγ causes an increase in the expression of Class II products on the membrane by increasing the mRNA levels of the cytoplasm. The mechanism of this increase is not known, although simple models, such as regulation by limiting the amount of invariant chain have been excluded. More importantly, the mechanism of coordinate or lack of coordinate regulation of the la genes is not yet known. The mechanics of la regulation both at the DNA and protein levels remain to be elucidated.