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Animals infected with strains of Trypanosoma brucei and T. rhodesiense exhibited cutaneous hypersensitivity to intradermal administration of antigen. This reactivity was of two types, an Arthus-type, antibody-mediated reaction which reached maximum intensity 4 hr after injection and a delayed-type, cell-mediated reaction which reached maximum intensity 24 hr after injection. This delayed-type hypersensitivity appeared in rabbits not earlier than 3 weeks after onset of infection. It did not occur in animals which received dead organisms. There was a cross-reaction in both types of reactivity between antigens prepared from T. brucei and T. rhodesiense . The delayed-type hypersensitivity was transferred to normal rabbits by intravenous inoculation of washed living cells from the spleen of a rabbit which showed delayed hypersensitivity. Dead cells failed to transfer hypersensitivity. The histological picture of the indurated area was typical of a delayed-type reaction.

Injection of lipopolysaccharide from Escherichia coli intravenously into mice affected the ability of their peritoneal macrophages to adsorb antisheep erythrocyte cytophilic antibodies in vitro. After a single intravenous injection of lipopolysaccharide, the alteration in antibody uptake took the form of a triphasic response. An early evanescent increase in adsorptive capacity took place, which reached a maximum 5 min after injection and was followed immediately by a phase of depressed uptake of antibody. Enhancement of antibody uptake then increased to reach a maximum about 24 hr after injection, before declining slowly. The second enhancement phase was inhibited by prior immunization with E. coli or after multiple injections of lipopolysaccharide. Exposure of macrophages to lipopolysaccharide in vitro did not alter their capacity to take up cytophilic antibody. Changes in the capacity of macrophages and polymorphonuclear neutrophil leukocytes to adsorb opsonized erythrocytes were induced by injection of lipopolysaccharide and were similar to those induced with respect to the uptake of cytophilic antibodies by macrophages.

Summary Antigenic studies on variants of Trypanosoma brucei isolated from infected rabbits at different intervals were carried out by agglutination and by opsonic adherence tests with corresponding antisera isolated from the same rabbits. Organisms isolated from infected rabbits 2 and 5 wk after infection were relatively inagglutinable by sera collected at the same time and 3 wk later from the rabbit from which the trypanosomes were derived. Experiments on adherence of trypanosomes to macrophages in the presence of antibodies indicated the lack of opsonic adherence particularly in the presence of a prozone. These results suggest possible antigenic loss in the presence of antibodies.

By using an in vitro test for phagocytosis, it was demonstrated that mouse antisheep erythrocyte macrophage-cytophilic antibodies did not significantly contribute to erythrocyte ingestion by mouse peritoneal macrophages.

The action of injected bacterial lipopolysaccharide upon the uptake of cytophilic antibodies by macrophages was found to be unrelated either to changes in surface charge or to the rate of anaerobic glycolysis of these cells. It is apparently mediated by alterations in the levels of material in the serum which enhances the uptake of cytophilic antibody by macrophages. This enhancing material does not appear to be an immunoglobulin nor can it be generated by the action of endotoxin on either whole blood or serum.