The Ly-4+ T Lymphocyte Subset in the Host Immune Response to the Asexual Stages of Plasmodium chabaudi chabaudi

T cells play a major role in acquired immunity to the asexual erythrocytic stages of malaria parasites. In different host/parasite combinations there is evidence that human CD4+ lymphocytes or their murine equivalent, Ly-4+, can act as helper cells in the production of protective Ab and also mediate cellular protective functions. The details of how the effector mechanisms operate in vivo, however, are not understood clearly. There is indirect evidence supporting an important role for Ly-4-bearing lymphocytes in the protective immune response to Plasmodium chabaudi chabaudi AS strain, a good animal model of P. falciparum infection. Experiments were performed to examine the nature of the Ly-4+ response to this parasite both in vivo and in vitro in order to characterise the cells responsible for the mediation of protective activity. Initial studies showed that during the course of a primary infection of P. c. chabaudi AS, there was a marked transient lymphocytosis in the peripheral blood, which occurred at a time just after peak parasitaemia (d 12-13 p. i. ). The adoptive transfer to syngeneic NIH recipients of either peripheral blood or splenic lymphocytes taken from donors at this early stage of infection conferred protection against homologous challenge. This was manifested in both competent and sublethally irradiated recipients as a reduced level and quickened remission of primary parasitaemia, and as a more rapid clearance of pRBC from the blood stream, compared to control mice receiving unprimed lymphocytes. Although it was possible to transfer immunity with preparations enriched for either T or B cells, optimal protection was conferred by an unfractionated population containing both lymphocyte phenotypes, suggesting that there was a degree of synergistic activity between parasite-primed T and B cells in the control of malarial infection. This concept was supported further by examination of serum Ab titres for recipients of semi-immune T, B or T & B spleen cells. In each instance, the level of specific anti-P. c. chabaudi AS Ig reached a peak between d 31-33 p. i. , at or just prior to recrudescence, but the highest titres were recorded for recipients of a mixed splenic population. Since serum Ig levels were quite low during the first wave of patent parasitaemia, it suggested that resolution of acute infection was achieved largely through Ab-independent mechanisms of immunity. This correlated well with a significantly quicker remission of primary parasitaemia observed in sublethally irradiated recipients of semi-immune T cells, compared to similarly treated mice receiving the same inoculum size of either B or T & B cells. To dissect further the protective immune response in this model, splenic T lymphocytes were taken from P. c. chabaudi AS strain-infected NIH mice on d 16 and d 20 of primary infection and after resolution of secondary and tertiary infections, and each of these preparations established as cell lines in vitro using a lysed extract of pRBC as the source of antigenic stimulation. All four lines were maintained in long term culture and all proliferated specifically in response to P. c. chabaudi AS Ag processed and presented by syngeneic APC. It was shown that recognition of the APC/Ag complex by T cells was an MHC class ll-restricted phenomenon, each cell line requiring APC of compatible H-2 haplotype for an in vitro proliferative response. By using surface immunofluorescence and the complement-mediated cytotoxicity assay, each line was characterised phenotypically as Ly-4+, i. e. belonging to the helper/inducer T cell subset. In vivo, adoptive transfer of each Ly-4+ line was effective in conferring protective immunity to naive and to immunocompromised mice. This was demonstrable, compared to controls given naive T and/or B cells, as both a reduced level and shortened duration of primary parasitaemia, and as a quicker parasite elimination. Inoculation of the P. c. chabaudi AS-reactive lines into non-immune mice challenged with genotypic or phenotypic variant pRBC indicated that there was a strain-specific element of the immunity transferred. Although mice were able to control infection with heterologous parasites, the greatest protection was conferred against challenge with the homologous pRBC to which the lines had been raised. For the two Ly-4+ lines taken from reinfected mice, the protective activity against P. c. chabaudi AS challenge upon adoptive transfer into adult-thymectomised, irradiated and bone marrow-reconstituted mice was improved significantly by the cotransfer of additional naive B cells. This suggested that these Ly-4-bearing lymphocytes act by Ab-mediated mechanisms in vivo. (Abstract shortened by ProQuest.)