Date of Award

1-1986

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Medical Sciences

Supervisor

Dr. William E. Rawls

Abstract

Two inbred strains of Syrian hamster have been shown to display genetically determined differences in resistance to infections with the arenavirus, Pichinde virus (PV). After intraperitoneal injection, the virus grows to higher titres in the spleens of the susceptible strain, MHA, than in the spleens of the resistant strain, LSH. Preliminary studies examining the basis of susceptibility demonstrated that resistance or susceptibility to the virus did not lie in an inherent difference in target cells to become infected, but suggested that there was a quantitative difference in target cells between the two strains of hamster. The following experiments were conducted in attempts to verify the hypothesis that MHA hamsters are susceptible to infection with Pichinde virus because they possess larger numbers of a splenic lymphocyte that serves as a target cell for virus replication and that also functions as an effector cell of nonspecific cytotoxicity.

The spleens and thymi of the high NK strain were found to display greater cellularity than those of the low NK strain. Additionally, thymocytes from MHA hamsters were found to proliferate to a greater extent than those of LSH hamsters in response to ConA-induced conditioned medium or purified interleukin 2 plus mitogen. As well, splenocytes from MHA hamsters showed high levels of lymphokine-activated killer cell (LAK) activity after culture in conditioned medium or interleukin 2. In both the thymus and the spleen, this difference in responsiveness was due to increased numbers of precursor cells responding to lymphokines in MHA organs. When lymphokine production was assessed, it was found that cells from the the high responder, MHA, synthesized less interleukin 2 than cells from LSH hamsters. Interleukin 1 production was equal in the two strains. These results led to the hypothesis that the susceptible hamsters contain immature lymphocytes, possibly because of the reduced interleukin 2 production. Increased numbers of relatively immature cells could account for the increased cellularity of lymphoid organs in these animals, and in the spleen, these cells may be responsible for increased NK activity, increased numbers of LAK precursors, and serve as target cells for PV replication.

Splenic cytotoxic cells in the hamster were characterized. Endogenous NK cells, virus-induced NK cells and LAK were all plastic nonadherent cells, as were the precursors for LAK. All populations expressed an antigen homologous to the murine Thy 1.2. Endogenous NK cells and virus-induced NK cells were similar in the expression of an asialo GMl homologue; both were reduced by 50% by treatment with this antiserum plus complement. LAK precursors and LAK effectors were negative for this marker, suggesting that LAK arise from the asialo GM1 negative component of endogenous NK activity. Treatment of hamsters with anti-asialo GM1 serum also reduced splenic NK activity in normal and virus-infected hamsters by 50%.

Cells infected with virus were characterized using the same criteria. A preferential infection of nonadherent cells was not evident before day 3 of infection, although MHA spleens already contained twice as much virus as LSH spleens. Both asialo GM1 negative and positive cells were infected. Culture of infected splenocytes in interleukin 2 induced high LAK, but failed to select out a population enriched for infectious centres compared to culture in medium alone, where no cytotoxic activity was evident. However, susceptible MHA hamsters with reduced NK activity after treatment with anti-asialo GM1 serum did display less virus at day 1 after infection with PV, but by day 3, virus loads were at control levels. Treatment with anti-asialo GM1 serum had no effect on the mortality of either strain of hamster. Treatment with purified interleukin 2 slowed mortality of MHA hamsters, although it did not do so by reducing viral replication. In total, t~ese data indicate that cells expressing an antigen detected by anti-asialo GMl serum that mediate some ~K ~ctivity can serve as target cells for PV replication, but these cells alone are not responsible. for the susceptibility of MHA hamsters • • Cytotoxic activity and infected centres could be dissociated, also suggesting that the jnitial hypothesis was incorrect. It could be that some other cell is the relevant target, that MHA hamsters may b~ susceptible because of increased number~ of all splenocyt~s, or that some .factor other than the presence or absence of a target cell accounts for their susceptibility.

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