Studies on human Ia supertypic specificities by complement fixation on PHA blasts. I: Serological properties of DC1 specificity

Summary A study of human Ia supertypic specificities associated with HLA-DR was done by the complement fixation microtechnique using PHA stimulated peripheral blood mononuclear cells as targets. Immune sera were obtained mostly from multi-transfused patients. Five sera strongly correlated with one another (r: 0.60 to 0.86) formed a cluster recognizing a specificity with a frequency of 0.658 in a caucasoid panel. It completely included HLA-DR2 and HLA-DRw6 specificities and partially included HLA-DR1. This specificity is analogous to DC1, MT1, MB1. In 38 families it segregated either DR1, 2, w6 or DR —. An anti-DC1 serum (Allaert) was studied in detail. An eluate was made from Daudi cell line (DRw6, DC1) sensitized by Allaert serum. Eluate reactivity against a panel of PHA blasts and lymphoid cell lines was identical to that of the serum. This indicates that both eluate and serum recognized DC1 specificity. Panel and absorption-elution studies showed that Allaert serum contained a strong anti-DC1 reactivity (titer 1:300) and a weaker anti-DR1 reactivity (titer 1:50). Both reactivities could be separated by absorption-elution, indicating that DC1 and DR1 specificities were not cross-reacting. Allaert serum reacted differently against PHA blasts and B lymphocytes (+ monocytes) from certain donors. Both targets from DR1 +, DC1 + donors were reactive (PHA+, B+). When cells from DR1 —, DC1+ donors were used, only PHA blasts were reactive (PHA+, B —). The results were confirmed by absorption-elution experiments. They suggested that both specificities were differently expressed on B lymphocytes and PHA blasts: DR1 being equally expressed on both targets, and DC1 being more quantitatively expressed on PHA blasts. Cross-absorption experiments with Allaert serum indicated that DC1 specificity was independent of DR specificities, suggesting distinct determinants for both specificities. PHA blasts from 21 DC1 positive donors were used for absorbing the serum reactivity against a single selected target (DR5, w6); 12 cells were able and 9 cells unable (or much less effective) to absorb the reactivity. Such results suggested a heterogeneity of DC1 specificity. This could be interpreted as DC1 being made up of several cross-reactive specificities with various absorbing capacities. The characteristics of DC1 specificity were comptible with the notion that it was carried by an Ia molecule distinct from the DR molecule. These results show that human Ia specificities can be studied by complement fixation on PHA blasts. The technique is potentially useful for characterization of (non DR) supertypic specificities.