Relationship between 4a and 4b HLA-determined specificities and C3 receptors of leukocyte membrane

The complement components of the major histocompatibility locus

Polymorphism of complement components, recognized by differences in either their antigenic specificity or their electrophoretic mobility, together with studies of inherited deficiencies, has enabled many of their structural genes to be mapped. In humans, three genes (for C2, C4, and factor B) have been placed between HLA-D and HLA-B on chromosome 6 and in mice, C4 between H2-I and H2-D, chromosome 17. Structural studies show that these components have exceptional features. C2 and factor B which contain the proteolytic active site of the C3 and C5 convertases are of the classical and alternative pathway respectively and are similar in structure and function. Both are novel types of serine proteases. C4 (as C3) contains an intrachain thioester bond essential for hemolytic activity. Molecular genetic investigations are determining the relative positions of these genes, and their precise structure, and should clarify their relation to the inherited diseases which are associated with defects in this section of the human genome.

Use of cell differentiation effectors to select a human B lymphoblastoid cell line enriched in C3b receptors

A study was undertaken to establish conditions of growth to increase C3b receptor synthesis on a human B lymphoblastoid cell line (Raji) by use of cell differentiation effectors. It appears that whereas two polar compounds HMBA (2mM) and Me2SO(2%) have no or little effect, 5 BrdU (30microM) and db cAMP (5 x10(-4) M) are able to increase in 48 h and 36 h respectively the synthesis of C3b receptor on Raji cell surface. These two compounds help to select a variant in which 100% of cells have C3b receptors with a high density of receptors per cell. The mechanism of BrdU action on the regulation of C3b receptor synthesis is discussed.

The blocking effect of antibodies against the products of the H-2 gene complex on lymphocyte complement (C3d) receptors. complement dependence and specificity

Antisera against the products of the major histocompatibility system (MHS), produced by immunization between congenic mouse strains differing only at that segment of the 17th chromosome rosettes which bears the H-2gene complex, exert a reproducible blocking effect on formation of with EACm (C3d rosettes). In many instances this effect is probably due to specific antibodies reactive with the lymphocytes, as indicated by immunochemical and adsorption experiments. Purified antibodies from the C3H anti-C3H.B10 immune ascites failed to exert this blocking effect. The blocking capacity can be restored, however, by addition of fresh normal mouse, rabbit or fetal calf serum, but not by heat inactivated serum. These experiments show that the presently defined H-2K, H-2D and H-2L antigens as well as some Ia antigens are themselves not the C3d receptors. The blocking effects observed in the presence of complement are possibly due to the proximity of H-2 or Ia molecules and C3d receptors, or to a rearrangement of membrane components after reaction with anti-MHC anti-MHC antibodies and complement.

Relationship between the HLA associated specifities 4a and 4b and the lymphocyte C3 receptor

Contrary to previous reports, we have obtained no evidence that 4a and 4b antisera specifically inhibit C3 rosette formation by human lymphocytes. Pretreatment of lymphocytes with various whole 4a and 4b antisera resulted in partial inhibition which was often nonsepcific. Ultracentrifugation of the sera to remove immune complexes removed C3 rosette inhibitory activity although specific cytotoxic activity remained. It is probable that immune complexes in antisera will have fixed C3 which will have been converted to C3d and so be able to block the C3d receptor. This is the receptor mainly measured in these and in the previous experiments.