Helper viruses associated with avian acute leukemia viruses inhibit the cellular immune response

Abstract

The cellular immune competence of lymphocytes obtained from birds infected with acute or chronic leukemia viruses was studied. The in vitro blastogenic response of thymus-derived lymphocytes (T cells) to the mitogens phytohemagglutinin (PHA) and connanavalin A (Con A) correlates with the ability of birds to elicit a cellular immune response ( C. K. Naspitz and M. Richter, 1968, Prog. Allergy 12, 1–85; P. Toivanen and A. Toivanen, 1973, J. Immunol. 111, 1602–1603). The PHA response of splenic lymphocytes from birds infected with the avian acute leukemia viruses, myelocytomatosis virus strain 29 (MC29), avian erythroblastosis virus (AEV), avian myeloblastosis virus (AMV), or reticuloendotheliosis virus (REV-T), was completely suppressed within 1 week after virus infection of chickens by avian acute leukemia virusus. Spleen cells from chickens infected with the chronic leukemia viruses, Rous-associated viruses (RAVs) types 1 and 3, exhibited PHA responses similar to those of uninfected birds. In contrast, lymphocytes from RAV-2 infected birds had suppressed PHA responses. Acute leukemia viruses are replication-defective and are associated with replication-competent nontransforming helper viruses. When chickens were infected with helper viruses isolated from the acute leukemia virus stocks by endpoint dilution, the T-cell response to various T-cell mitogens was completely suppressed. The helper viruses, therefore, contribute to the immunosuppression which occurs during the development of avian acute leukemia. The rapid lethality of the acute leukemia viruses could be, in part, the result of the immunosuppressive activity of the associated helper viruses. Though all the acute leukemia viruses severely immunosuppress their hosts, the mechanism by which the immunosuppression was induced by these viruses differed. REV-T was the only acute leukemia virus which induced a population of splenic suppressor cells. All the viruses of the leukosis-sarcoma complex tested which had subgroup B specificity were immunosuppressive while those of subgroup A were not. These results suggest that immunosuppression by these viruses may be related to subgroup specificity.