Interferon

The importance of interferon-a in defence against CMV has been demonstrated in the murine model of CMV infection, where the susceptibility of mice to infection is highly strain dependent. It was noted that after inoculation with murine CMV, higher plasma levels of interferon were induced in the resistant CBA than in susceptible Balb/C mice (213). Furthermore, the treatment of the resistant CBA mice with

antiserum to interferon-a significantly reduced their resistance to murine CMV infection. (214). However, interferon-independent factors apparently also played a role in determining the resistance to murine CMV, since the treatment of susceptible Balb/C mice with large doses of interferon-a failed to protect them from lethal infection (214).

The importance of interferon-y in viral infections is illustrated by the comparison of lymphocytic choriomeningitis virus (LCMV) and murine CMV infection, where C57BL/6 mice infected with murine CMV were susceptible to NK cells, but those infected with LCMV were not. Both infections induced type I interferons and the cytolytic capacity of NK cells, however, interferon-y was produced only in response to infection by murine CMV and not LCMV (215). The importance of interferon-y in providing early protection from murine CMV infection was demonstrated by the decreased survival of infected mice that were treated with anti-interferon-y antibody.

1.5.2 NK cells

The importance of NK cells in CMV was highlighted by the case of a patient devoid of NK cells, who exhibited an extreme susceptibility to herpesvirus infections, including CMV (216). The adolescent patient presented with varicella virus infection that was successfully treated, but then she succumbed a few years later to life threatening CMV infection with interstitial pneumonia. CMV was isolated from the lung, blood and urine of the patient. Further investigation revealed a complete lack of NK cells and NK cell activity, even after in vitro induction with interferon or IL-2. However, her immune function including specific T-cell and antibody responses was otherwise normal (216). Such selective deficiencies in natural killer cells are rare. For this reason it has been difficult to assign a definitive role for NK cells in the in vivo defence against CMV in humans. Other positive correlations between depressed NK cell function and sensitivity to CMV infection have been noted in a number of patient groups. A reduced number of natural killer cells were detected in the peripheral blood of children with symptomatic congenital CMV infection compared to children who experienced an asymptomatic infection (217). In addition, peripheral blood mononuclear cells from the asymptomatic group displayed increased lysis of CMV-infected fibroblasts. This suggested that the increased

number and activity of NK cells played a role in preventing severe disease in these patients (217). NK cells are the first cells to reappear after bone marrow transplantation and are thought to play a role in the protection against CMV, particularly during the first three months after the transplant, which coincides with the period in which patients are most susceptible to infection. A role for NK cells in these patients was supported by studies that demonstrated that the capacity of bone marrow transplant recipients to develop an NK cell cytotoxic response, as measured by the lysis of K562 tumour cells, correlated with each patient’s ability to recover from CMV infection (218,219). In a later study, patients who developed CMV disease had significantly lower NK cell cytotoxic responses against CMV- infected fibroblasts than patients who did not develop CMV disease (220). The CMV-infected cells used in the latter study are likely to be a more relevant choice of target cells compared to the K562 tumour cells used in the earlier studies. Further support of a role for NK cells in the recovery from CMV infection has come from studies of renal transplant recipients. Spontaneous recovery from primary CMV infection coincided with a significant increase in the number of activated NK cells, as measured by the expression of H1_A-DR on these cells (221). A later study from the same group showed that NK cell activity, measured in a cytotoxicity assay against K562 tumour target cells, also correlated with the recovery from CMV disease (222). Conversely, NK cell activity has been shown to be lower in renal transplant patients with severe CMV disease compared to those having asymptomatic or mild infection (223).

The most persuasive evidence demonstrating the importance of NK cells in the regulation of viral infections has been provided by in vivo studies using the murine model for CMV. The groups of Welsh et al. and others have generated solid evidence showing that NK cells limit the severity, extent and duration of acute murine CMV infection. Mice depleted of NK cells by using NK cell-specific antibodies, have enhanced virus replication in the spleen, lung and liver after inoculation with murine CMV (224,225). It appeared that NK cells exerted their antiviral effect early in the infection, since their depletion later in infection had no effect on virus titres (224). Suckling mice are highly sensitive to murine CMV infection, and continue to be so until the time at which their NK cell responses have

matured. The adoptive transfer of mature NK cells protected these young mice from lethal murine CMV infection (226). In addition, cloned NK cells provided resistance to murine CMV in irradiated adult mice (226).

Mice with severe combined immunodeficiency are devoid of acquired immune responses, and therefore provide an ideal model to unambiguously study the role of NK cells in viral infection. In these mice, NK cells inhibited murine CMV replication, although they were unable to eradicate the infection completely, indicating that T and B cells were also necessary for the resolution of the infection (225,227). These results were consistent with the concept that NK cells inhibit the spread of infection, thus allowing time for specific immune responses to develop.

In contrast to the above results for murine CMV, infection of mice by LCMV is not controlled by NK cells, even though NK cells were present in similar numbers and had equivalent cytotoxic activity (228). Furthermore, both viruses induced type I interferon, however, it was found that the infected cells displayed interferon- dependent differences in their susceptibility to lysis by NK cells (228). Interferon pre-treatment of uninfected mouse fibroblasts, or those infected with LCMV made them resistant to lysis by activated NK cells. However, after interferon treatment, murine CMV-infected fibroblasts remained susceptible to lysis. Thus, virally induced interferon protected LCMV-infected cells from lysis by NK cells, but not those infected with murine CMV, thereby possibly contributing to the difference in NK susceptibility of these two viral infections.