Delayed (footpad) hypersensitivity and Arthus reactivity using protein-rich antigens and LPS in mice immunized with live attenuated aroA salmonella vaccines

Abstract

Footpad reactions to protein-rich salmonella extracts and lipopolysaccharide (LPS) were studied in BALB/c mice 2 and 8 months after immunization with the Salmonella typhimurium SL3261 aroA live vaccine. T-cell depletion in vivo and adoptive serum transfer showed that protein-rich antigens induced T-cell dependent delayed hypersensitivity reactions, whereas LPS only elicited Arthus reactions. The footpad reactions to crude protein extracts were not always T-cell mediated, but depended on the nature and the dose of the antigen. Selective depletion of CD4 + T cells alone had a greater effect than depletion of CDa + T cells alone, but neither was as marked as simultaneous depletion of both CD4 + and CD8 + T cells, which abolished the delayed type hypersensitivity (DTH) response. Crude protein-rich extracts subjected to alkaline hydrolysis (which removes some ester-linked fatty acids and causes disaggregation of LPS resulting in decreased toxicity while conserving O-specificity) still gave positive T-cell dependent reactions, but with reduced T cell independent reactivity. Purified phenol-water LPS (2.5 μg) produced Arthus reactivity which could be confused with DTH. LPS induced positive reactions which still occurred in T-cell depleted mice and were transferable by immune serum. Arthus reactions did not occur when using alkali-treated LPS, which showed reduced complement fixation in vitro when using serum from immunized mice. The results indicate that footpad testing using salmonella antigens containing LPS elicit DTH but can also produce toxic reactions, some of which are T-cell independent and not necessarily a true measure of DTH. Arthus reactivity to LPS can be confused with DTH. Alkaline hydrolysis of the antigens can eliminate non-specific reactogenicity while retaining the ability of the (protein-rich) antigen to elicit a true T-cell dependent footpad response, which requires the participation of both CD4 + and CD8 + T cells.