Is the delayed-type hypersensitivity observed after a low dose of antigen mediated by helper T cells?

In mice receiving, i.v., a dose of antigen optimal for antibody response, no delayed-type hypersensitivity (DTH) reaction is detectable. In contrast, in mice receiving a dose of antigen too small to induce B cell activation, a DTH reaction is elicitable shortly and transiently after immunization. Using a sensitive titration assay of DTH-mediating T lymphocytes, this reciprocal relationship between antibody production and DTH responses was reinvestigated. The absence of peripheral DTH reactivity in mice primed i.v. with a high dose of antigen (10(9) heterologous red blood cells) does not result either from the absence of activation and clonal expansion of DTH-mediating cells or from induction of suppressive mechanisms but results from a decreased circulation of DTH-mediating cells. The present studies show that DTH-mediating cells disappear from blood to enter the spleen only when specific B lymphocytes are present and activated by a high dose of antigen. These results are compatible with the hypothesis that T cells activated by antigen can function either as helper cells for B lymphocytes or as DTH-mediating cells, depending on the environment they reach during their migration. In order to demonstrate that the same cell may support the two functions, monoclonal T lymphocytes were assayed for their helper function and for their ability to transfer a DTH reaction.