We have recently shown that human monocytes and U937 cells possess two molecular classes of Fcy receptor. One, a 72,000-mol-wt sialoglycoprotein, has high affinity for certain subclasses of human and murine monomeric IgG. The other is a 40,000-mol-wt protein (p40) with low affinity for monomeric IgG but with the capacity to bind IgG aggregates or IgG-coated particles.In the present study, a 40,000-mol-wt single chain protein, apparently identical to p40 from U937 cells, was isolated from surface-radioiodinated human platelets by affinity purification using a murine IgG2b monoclonal antibody to p40. This 40,000-mol-wt protein was not seen when control IgG2b or unrelated murine monoclonal antibodies were employed in place of antip40. The same 40,000-mol-wt protein was also recovered from an IgG-Sepharose affinity adsorbent, but not from ovalbuminor myoglobin-Sepharose. The 72,000-mol-wt Fcvy receptor of monocytes was not identified on platelets.Monoclonal anti-p4O and Fab fragments derived from this antibody blocked platelet aggregation by heat-aggregated human IgG, whereas a control murine IgG2b protein had little or no inhibitory effect at 500-1,000-fold higher concentrations. A murine IgG1 monoclonal antibody, reactive with an unrelated platelet-specific membrane antigen, did not inhibit platelet responses to aggregated IgG. Anti-p4O did not affect platelet aggregation by thrombin, collagen, or fibrinogen plus ADP. Although antip40 did not directly aggregate platelets in the concentrations employed, cross-linking with F(ab'`) goat anti-murine Ig induced apyrase-sensitive aggregation of anti-p40-treated platelets. This indicates that p40 possesses transmembrane linkage for platelet activation and secretion.These observations strongly suggest that this newly recognized 40,000-mol-wt platelet membrane protein serves as an Fcy receptor.
Immunoglobulin G (IgG) autoantibodies of 20 patients with autoimmune hemolytic anemia (AHA) were used in immunoaffinity assays with surface-radioiodinated human red was not seen in the absence of p34, and both proteins are likely to be members of the Rh family. Indeed, a 34-kD polypeptide band and 37-55-kD polydisperse "smear," isolated concurrently from the same labeled RBCs by IgG allo-anti-e, were indistinguishable from their autoantibody-isolated counterparts and may well be the same protein identified at different epitopes by the auto-and allo-antibodies. Individual AHA patients' autoantibodies isolated p34 and gp37-55, alone or in combination with band 3 (nine cases); strong band 3 alone (five cases); and combinations of band 3 with GPA (six cases). The autoantibodies of three additional patients whose AHA had been induced by a-methyldopa also isolated p34 and gp37-55. (J. Clin. Invest. 1993. 91:1672-1680
A B S T R A C T The first recognized human kindred with hereditary deficiency of the fifth component of complement (C5) is described. The proband, a 20-year-old black female with systemic lupus erythematosus since age 11, lacked serum hemolytic complement activity, even during remission. C5 was undetectable in her serum by both immunodiffusion and hemolytic assays. Other complement components were normal during remission of lupus, but C1, C4, C2, and C3 levels fell during exacerbations.A younger half-sister, who had no underlying disease, was also found to lack immunochemically detectable C5. By hemolytic assay, she exhibited 1-2% of the normal serum C5 level and normal concentrations of other complement components. C5 levels of other family members were either normal or approximately half-normal, consistent with autosomal codominant inheritance of the gene determining C5 deficiency.Normal hemolytic titers were restored to both homozygous C5-deficient (CSD) sera by addition of highly purified human C5. In specific C5 titrations, however, it was noted that when limited amounts of C5 were assayed in the presence of low dilutions of either C5D serum, curving rather than linear dose-response plots were consistently obtained, suggesting some inhibitory effect. Further studies suggested that low dilutions of C5D serum contain a factor (or factors) interfering at some step in the hemolytic assay of C5, rather than a true C5 inhibitor or inactivator.
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