Degree Name

Doctor of Philosophy


Department of Biology


Monocytes were isolated from the peripheral blood of healthy adult donors, seronegative for human cytomegalovirus (HCMV), and challenged in vitro under mycoplasma-free conditions with a laboratory-adapted (AD169) or a clinical (p72) strain of HCMV . The ability of challenged monocytes to function as efficient immune accessory cells for antigen presentation and cytokine secretion was compared to that of control, mock-challenged monocytes. HCMV-challenged monocytes demonstrated a reduced ability to present antigen to autologus T cells and expressed decreased levels of MHC class II HLA-DR molecules particularly in the presence of augmenting stimuli such as interferon gamma. It is proposed that the drop in HLA-DR expression contributed to decreased antigen presenting ability. Supernatants harvested from HCMVchallenged monocytes contained suppressor factors. On separation, two suppressor fractions were obtained which were suppressive for antigenspecific T cell proliferation and for the T cell mitogenic response. Inhibitory activity was detected in fractions of molecular weight 130-180 kDa from culture supernatants of both control and virus-challenged cells, but the levels were elevated in the latter. In addition there was a novel suppressor activity in the fraction of molecular weight 75-100 kDa from the virus-challenged cultures. This suppressive fraction specifically inhibited T cell interleukin 2 (II2) responsiveness, decreasing the detectable levels of the low affinity p55 (TAC/α chain) component of the high affinity II2 receptor (II2R). T cells exposed to this fraction retained the capacity to respond to IL2, only when the lymphokine was added at high levels, apparently through the selective retention of the intermediate affinity, p75 (β chain) component of the IL2R. HCMV-challenged monocyte supernatants also contained elevated amounts of interleukin 1 (IL1), and tumour necrosis factor alpha (TNF α), while levels of interleukin 6 (IL6) remained unchanged. At times later than 48h post virus challenge, increased IL1 levels (as recorded by ELISA) could not be detected in an IL1 biological activity assay due to the masking influence of the suppressor factors. Because IL1 and TNF α are potent mediators of the inflammatory response, elevated levels of these cytokines induced in response to HCMV in vivo could be associated with the immunopathology associated with HCMV infection. Production of suppressor factors active against the T cell proliferative response could provide a means whereby HCMV has a general effect on cell mediated immunity and causes generalized immunosuppression in vivo.