The Q7 α3 domain alters T cell recognition of class I antigens

In this study we have analyzed the role of the α3 domain of class I molecules in T cell recognition. Using the laboratory engineered molecules LLQQ (α1/α2 from L(d), α3, and phosphatidyl inositol (PI) linked C terminus from Q7) and LLQL (α1/α2 from L(d), α3, from Q7, transmembrane (TM) and cytoplasmic domains from L(d)) we show that these molecules are not recognized by primary L(d)-specific CTL. The cell membrane expression of both L(d) and LLQL are upregulated by co-culture with an exogenously supplied murine cytomegalovirus-derived peptide indicating that the Q7 α3 domain does not interfere with binding of Ag to α1/α2. However, only peptide pulsed L(d) but not LLQL target cells are recognized by L(d)-restricted-peptide specific CTL. In contrast to the above results, LLQL and LLQQ molecules can be recognized by bulk alloreactive anti-L(d) CTL and 2/3 of CTL clones derived from in vivo primed mice. The fact that these secondary CTL recognize LLQQ indicates that a PI linkage is permissive for presentation of class I epitopes to alloreactive CTL. These secondary CTL are resistant to blocking at the effector stage by mAb against CD8 and express relatively low levels of membrane CD8 molecules compared to CTL from unprimed mice. Further, culture of unprimed CTL precursors in the presence of CD8 mAb also allows for the generation of CD8-independent CTL that recognize LLQL. Taken together, these data indicate that the α3 domain of Q7 (Qa-2) prevents CD8-dependent CTL from recognizing L(d), regardless of whether the class I molecule is attached to the cell surface by a PI moiety or as a membrane spanning protein domain. We hypothesize that this defect in recognition is most likely due to an inability of CD8 to interact efficiently with the Q7 α3 domain and could account for why Q7 molecules do not serve as restricting elements for virus and minor H-Ag-specific CTL.