Live lactate dehydrogenase-elevating virus (LDV) induces suppressor T cells that inhibit the development of delayed hypersensitivity to LDV.

A strong delayed-type hypersensitivity (DTH) response to lactate dehydrogenase-elevating virus (LDV) in mice was induced by immunization with u.v.- or heat-inactivated virus by the intravenous, intraperitoneal and especially the subcutaneous route. The peak DTH response was seen 7 days after immunization. Live virus, in contrast, did not induce a DTH response in young (1- to 3-month-old) mice irrespective of inoculation route, except after pretreatment with cyclophosphamide (CY), when the response peaked at 14 days. Live virus, however, induced a significant DTH response in old mice (greater than 8 months) without CY. The DTH response to inactivated virus was reduced when live virus was given intraperitoneally at the same time, but was partially restored when Ia-positive peritoneal cells were also given intravenously. The DTH response induced in infected mice pretreated with CY was suppressed by injection of spleen cells from infected or from normal 1- to 3-month-old mice. Suppression by normal spleen cells was abolished by treatment with anti-Thy1.2, anti-Lyt1 and anti-Lyt2 plus complement, whereas suppression by spleen cells from infected mice was prevented by anti-Thy1.2 and anti-Lyt2 plus complement. It is concluded that suppression of the DTH response was associated with induction of suppressor T cells and that severe depletion of Ia-positive cells may also have played a part.