Inhibition of Immune Phagocytosis by Human Sera with HLA A, B, C and DR but Not with DQ or EM Type Reactivity

613 sera from pregnant women were investigated for inhibition of immune phagocytosis (IPI) by monocytes exposed to anti-D (Rh)-sensitized human red blood cells. IPI was detected in 42 (26%) of 165 and in 78 (17%) of 448 sera assessed against monocytes from 15 or 5 panel donors, respectively. All IPI-positive sera reacted in an allotypic pattern. Eight IPI-positive sera tested with autologous monocytes were found to be nonreactive. Upon comparative analysis of 448 sera, a significant correlation was found between the specific patterns of IPI and lymphocytotoxicity by HLA antibodies. In addition, 13 sera (4%) were positive for IPI, but not for lymphocytotoxicity. Twenty IPI-positive sera were tested by indirect immunofluorescence against platelets and T lymphocytes and revealed IgG antibodies in 16 and 11 sera, respectively. IPI activity could be removed from 8 out of 10 positive sera by platelet pool absorption. While virtually all sera exhibiting HLA, A, B or C-like activity (cytotoxicity with all cells) or HLA DR-like activity (exclusive cytotoxicity with B lymphocytes and monocytes) were IPI-positive, IPI was infrequently observed with sera containing HLA DQ-like activity (cytotoxicity with B lymphocytes only). IPI was also rarely seen with sera cytotoxic only to monocytes and not at all with sera containing antibodies against endothelial/monocyte antigens. We conclude that IPI is caused by cytotoxic as well as noncytotoxic HLA A, B, C, DR-specific antibodies. This effect may bear significance for the maternal immune response against fetal antigens and may be useful for pretransplant histocompatibility testing.