All the clinical and pathologic features of malaria are solely attributable to the parasitic stages of the asexual erythrocytic cycle. The propagation of this cycle in a host is dependent upon extracellular merozoites attaching to and invading susceptible erythrocytes . This attachment and initiation of invasion by malaria merozoites is mediated through specific interactions between parasite receptors and ligand molecules on the erythrocyte plasma membrane (1, 2). The identification oferythrocyte liganns used by invading merozoites is important to understanding this biologic event at the molecular level. Furthermore, this knowledge can aid in the isolation ofmalaria parasite receptor antigens ; potential targets of vaccine induced immunity.Studies on Plasmodium knowlesi, a simian malaria that can invade human erythrocytes, have indicated that an erythrocyte membrane component associated with Duffy blood group determinants is used by this species as a ligand for merozoite invasion (3-5). These studies on a simian malaria became more relevant to human malaria when indirect and retrospective studies in vivo also correlated susceptibility to infection by Plasmodium vtvax, a human malaria parasite of major importance, with the presence of the Duffy blood group antigens (6, 7).Direct ligand studies with P. vivax could not be done, however, because these parasites were difficult to obtain and could not be cultured in vitro, as has been possible with Plasmodium falciparum and P. knowlesi . Here we describe a short term invasion assay in vitro for P. vivax parasites obtained from squirrel (Saimiri sciureus) monkeys. We have made use of this invasion assay to investigate the role of the Duffy blood group as an erythrocyte ligand for P. vivax merozoites using human and simian erythrocytes of varying Duffy phenotypes and a mAb against a new Duffy blood group determinant, Fy6 (8). Materials and MethodsParasites and Cultures. P. knowlesi (H strain) parasites were cryopreserved in liquid nitrogen as ring stage infected erythrocytes from rhesus monkeys (9).

The Duffy blood group system of human erythrocytes comprises a membrane glycoprotein recently characterized by immunoprecipitation techniques (1). It is a polymorphic system that includes determinants for five alloantigens, two of which are Mendelian alleles (Fya and Fyb) (2, 3). A null genotype, designated Fy(a b-) (4), lacks the expression of both Fya and Fy b, and is inherited as a third allele of the series . Homozygous Fy(a-b-) red cells, however, also lack expression of the Fy3 (5) antigen, which is present on all other Duffy types. Fy4 (6) occurs on almost all samples of Fy(a b-), but the expression is weak on some . In addition Fy4 is occasionally found on red cells of Fy(a +b'), so that its exact Duffy status is not in perfect agreement with the absence of both Fya and Fy b (7). Fy5 (8) resembles Fy3 but is also absent from Rh-null red cells regardless of their other Duffy types.These five specificities are inherited en bloc but they differ in their susceptibility to proteolytic enzyme treatment; the high frequency antigens Fy3, Fy4, and Fy5 being resistant to the effects of chymotrypsin, ficin, and papain, while Fya and Fy b are completely destroyed by these enzymes (7). Duffy maps to chromosome 1 (9) (i.e., is syntenic to Rh), which may be related to the peculiar absence of Fy5 on Rh-null cells.Interest in the Duffy system has been considerable since the demonstration by Miller et al . (10) We have prepared a murine monoclonal antibody, anti-Fy6, that reacts with all human red cells except for those of the Fy(ab-) phenotype. The specificity defined by anti-Fy6 differs significantly from others previously identified in the Duffy system . In this report we show that Fy6 is not only characteristic of Duffy, but is related to the penetration of red cells by P. vivax merozoites. 776J. Exp. MED.

Fifteen antigenic determinants are known to be related to the Kell blood group. Some boys with X-linked chronic granulomatous disease have the very rare McLeod or Ko phenotype on their red cells. Serological studies of the McLeod type suggest that the weak Kell antigens that are present differ qualitatively and quantitatively from those on red cells of common Kell type. A new antigen, Kx, has been characterized and shown to be present on red cells and neutrophil leucocytes. Lack of red-cell Kx is associated with the McLeod phenotype, lack of leucocyte Kx is associated with chronic granulomatous disease.

Anti-A and anti-K have been found in the serum of a 20-day-old child who had not been transfused but who was acutely ill with E. coli enterocolitis. Both antibodies are IgM proteins. The mother's serum does not contain either antibody and the anti-A and anti-K in the infant's serum are not of maternal origin. Both parents and the child are of the Kell phenotype K-k+. Stool cultures made from the child yielded E. coli O 125:B15, an uncommon B-variant pathogenic coliform. Cell-free preparations made from broth cultures of this organism have strong specific inhibitory activity against IgM anti-A and anti-K, and both antigens have been identified on the bacterial cells. At age 3 months the child had made a clinical recovery, stool cultures showed no pathogenic coliforms, and anti-A and anti-K were no longer detectable in her serum. These data indicate that absorption of metabolites with A-like and K-like activity produced by a pathogenic coliform in the intestinal tract were responsible for the appearance of apparent naturally occurring anti-A and anti-K in the child's serum.