Class II major histocompatibility complex encoded proteins (MHC class II or Ia molecules) are principal plasma membrane proteins involved in activation of both B and T cells during antigen-driven immune responses. Recent data indicate that class II molecules are more than simply recognition elements that provide a ligand for the T cell antigen receptor. Changes in B cell physiology that follow class II binding are now recognized as being required not only for the induction of T cell activation, but also for B cell activation and proliferation. It is interesting to note that class II molecules appear to transduce signals via two distinct mechanisms depending upon the differentiative state of the B cell on which they are expressed. While one of these pathways, involving cAMP generation and protein kinase C localization in the cytoskeletal/nuclear compartment, is seen in resting B cells, the second is seen in primed B cells and involves tyrosine kinase activation, inositol lipid hydrolysis, and Ca2+ mobilization. Use of this pathway is correlated with ability of class II to transduce signals leading to B cell proliferation. To begin to address the molecular basis of this unique, activation-dependent, differential coupling of class II to signaling pathways, we conducted mutational analysis of class II structural requirements for signal transduction. Here we report that the cytoplasmic (Cy) domains of I-Ak class II molecules are not required for either receptor-mediated activation of protein tyrosine phosphorylation or Ca2+ mobilization. This is in contrast to the requirement of the Cy domain of beta chain of class II for the alternate signaling pathway and efficient antigen presentation to autoreactive T cell lines. Disparate distribution of functional motifs within the MHC class II molecules may reflect use of distinct receptor associated effector molecules to sustain different modes of signal transduction in various class II-expressing cells.

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