Interdependence of CD4⁺ T cells and malarial spleen in immunity to plasmodium vinckei vinckei. Relevance to vaccine development

We studied immunity to the blood stage of the rodent malaria, Plasmodium vinckei vinckei, which is uniformly lethal to mice. BALB/c mice develop solid immunity after two infections and drug cure. The following experiments define the basis of this immunity. Transfer of pooled serum from such immune mice renders very limited protection to BALB/c mice and no protection to athymic nu/nu mice. Moreover, B cell-deficient C3H/HeN mice develop immunity to P. vinckei reinfection in the same manner as immunologically intact mice, an observation made earlier. In vivo depletion of CD4⁺ T cells in immune mice abrogates their immunity. This loss of immunity could be reversed through reconstitution of in vivo CD4-depleted mice with fractionated B^-, CD8^-, CD⁴⁺ immune spleen cells; however, adoptive transfer of fractionated CD4⁺ T cells from immune spleen into naive BALB/c or histocompatible BALB/c nude mice does not render recipients immune. In vivo depletion of CD8⁺ T cells did not influence the parasitemia in nonimmune or immune mice. Splenectomy of immune mice completely reverses their immunity. Repletion of splenectomized mice with their own spleen cells does not reconstitute their immunity. We conclude that some feature of the malaria-modified spleen acts in concert with the effector/inducer function of CD⁴⁺ T cells to provide protection from P. vinckei. To be consistent with this finding, a malaria vaccine may require a combination of malaria Ag to induce immune CD⁴⁺ T cells and an adjuvant or other vaccine vehicle to alter the spleen.