Effects of L3T4+ lymphocyte depletion on acute murine cytomegalovirus infection

Susceptibility to cytomegalovirus infection may be dependent on the cytokine response to the virus

BACKGROUND

Cytomegalovirus (CMV) infection is an important cause of morbidity and mortality in an immunocompromised host. Pulmonary infection with CMV results in an inflammatory response, which includes the local production of cytokines. Cytokine production stimulated by CMV infection serves to activate a series of immunologic responses involved in viral clearance. Previous work has demonstrated that different mouse strains express variable sensitivity to CMV infection.

METHODS

Using mouse strains that express sensitive (BALB/cj) and resistant (C57BL/6) CMV phenotypes, we asked whether the differences in susceptibility to infection were caused by differences in pulmonary cytokine production after intraperitoneal infection with CMV.

RESULTS

C57 mice demonstrated a higher total bronchoalveolar lavage (BAL) and BAL lymphocyte count at 3 and 7 days after intraperitoneal infection compared with BALB mice. There were no differences in BAL cytokine production; however, we were able to demonstrate differences in CMV DNA load in the lungs of BALB mice compared with that of C57 mice. In addition, there appeared to be increased whole-lung production of the TH2 cytokine IL-10 in the BALB mice versus the C57 mice.

CONCLUSIONS

This observation suggests that the genetic susceptibility to CMV infection may, in part, be regulated by differences in cytokines production within the local environment.

Requirement for natural killer cell-produced interferon gamma in defense against murine cytomegalovirus infection and enhancement of this defense pathway by interleukin 12 administration

The presence of natural killer (NK) cells contributes to early defense against murine cytomegalovirus (MCMV) infection. Although NK cells can mediate in vivo protection against MCMV, the mechanism by which they do so has not been defined. The studies presented here evaluate cytokine production by NK cells activated during MCMV infection and the role of NK cell-produced cytokines in early in vivo antiviral defenses. Experiments with normal C57BL/6, T cell-deficient C57BL/6 nude, and severe combined immunodeficient mice lacking T and B cells demonstrated that both interferon gamma (IFN-gamma) and tumor necrosis factor (TNF) production were induced at early times after infection with MCMV. Conditioned media samples prepared with cells from these mice, on day 2 after infection, produced 11-43 pg/million cells of IFN-gamma and 12-19 pg/million cells of TNF as evaluated by specific protein enzyme-linked immunosorbent assays. Studies in the NK- and T cell-deficient mouse line, E26, in mice that had been depleted in vivo of NK cells by treatment with antibodies eliminating NK cells, anti-asialo ganglio-N-tetraosylceramide or anti-NK1.1, and with populations of cells that had been depleted of NK cells by complement treatment with the anti-NK cell antibody, SW3A4, demonstrated that NK cells were solely responsible for the IFN-gamma but were not required for TNF production. The in vivo absence of NK cells was accompanied by increased viral hepatitis and viral replication in both immunocompetent and immunodeficient mice, as well as decreased survival time of immunodeficient mice. In vivo treatments with antibodies neutralizing IFN-gamma demonstrated that this factor contributed to the NK cell-mediated antiviral defense and reduced the measured parameters of viral defense to levels indistinguishable from those observed in NK cell-deficient mice. These effects appeared to be independent of cytolytic activity, as NK cells isolated from anti-IFN-gamma-treated mice mediated killing at levels comparable to those observed in control-treated mice. The consequences of interleukin 12 (IL-12) administration, a known potent inducer of IFN-gamma production by NK cells, were evaluated in MCMV-infected mice. Low IL-12 doses, i.e., 1 ng/d, increased NK cell cytotoxicity and IFN-gamma production up to twofold and resulted in improved antiviral status; virus-induced hepatitis was decreased as much as fivefold, and viral burdens were decreased to levels below detection.(ABSTRACT TRUNCATED AT 400 WORDS)

Analysis of the role of CD4+ T-cells during murine cytomegalovirus infection in different strains of mice

We examined the role of CD4+ T-cells in peritoneal exudate cells (PECs) during the course of acute murine cytomegalovirus (MCMV) infection in two strains of mice. Cell counts of PECs and cytofluorometric analysis showed that C57BL/6, a resistant strain, had more CD4+ T-cells than BALB/c, a susceptible strain, after intraperitoneal infection of 3 x 10(3) PFU of the Smith strain of MCMV, though both strains had an equivalent number of CD8+ T-cells. CD4+ T-cells of both strains expressed mRNA of IFN-gamma, IL-2, and IL-4 on days 5 and 7 after infection, with much higher expression of these cytokines in C57BL/6 than in BALB/c. At the same time point after infection, macrophages were shown to express mRNA of IL-1 alpha and TNF-alpha with higher expression of IL-1 alpha in C57BL/6 than in BALB/c. Production of nitric oxide, recently shown to be one of the antiviral effector mechanisms of macrophages, by macrophages of both strains was examined showing more production of nitric oxide on day 7 after infection in C57BL/6 than in BALB/c. From these findings, we suggest the possibility that CD4+ T-cells contribute to the protection against MCMV infection via the secretion of cytokines and the resultant activation of macrophages to produce nitric oxide.

Correlation between chemical suppression of natural killer cell activity in mice and susceptibility to cytomegalovirus: rationale for applying murine cytomegalovirus as a host resistance model and for interpreting immunotoxicity testing in terms of risk of disease

The purpose of this study was to determine the relationship between chemical suppression of natural killer (NK) cell activity in mice and chemical effects on susceptibility to murine cytomegalovirus (MCMV) infection. The goal was to provide a rational basis for applying MCMV as a host resistance model for immunotoxicity testing and to provide risk assessors some guidance in relating suppression of NK cell activity to enhanced risk of disease. Data from studies with eight chemicals administered in various doses and by various routes were evaluated, and a significant correlation was observed between chemical suppression of virus-augmented NK cell activity and increased mortality due to MCMV infection. In contrast, effects of the same chemical treatments on spontaneous NK cell activity (i.e., basal activity in uninfected mice) did not correlate with effects of these chemicals on mortality due to MCMV. Although chemicals that suppressed spontaneous NK cell activity enhanced infection, the converse was not always true--that is, increased susceptibility to infection and suppression of virus-augmented NK cell activity were observed on three occasions when spontaneous NK cell activity was unaffected. This latter phenomenon plus the fact that for two chemicals spontaneous NK was suppressed at concentrations twofold below that which affected mortality appear to account for the poor statistical correlation. Nevertheless, the data indicate that MCMV is a useful host resistance model to be applied in immunotoxicity testing when suppression of NK cell activity has been demonstrated. However, virus-augmented activity may be a better indicator than spontaneous activity. The data also indicated that suppression of NK cell activity is predictive of increased susceptibility to infection and hence provides qualitative guidance (hazard identification) to risk assessors.

T, not B, lymphocytes are required for immunodeficiency and paralysis induced by ts1, a mutant of Moloney murine leukemia virus-TB

BALB/c mice when injected as newborn with ts1, a temperature-sensitive mutant of Moloney murine leukemia virus-TB, developed a fatal hindlimb paralysis and immunodeficiency. This disease induction was prevented, to a great extent, by transient depletion of the peripheral T lymphocytes during the early course of infection by using anti-Thy 1.2 antibody. FVB/N mice, which are highly susceptible to ts1, but express Thy 1.1 instead of Thy 1.2 on their T lymphocytes, did not show any difference in the disease profile when treated similarly with anti-Thy 1.2 antibody. Transient depletion of the peripheral B lymphocytes in BALB/c mice in the early course of ts1 infection had no effect on the disease induction. In the T cell depleted BALB/c mice, virus replication was reduced, survival of the mice was increased and viral specific antibodies were produced, whereas, in the B cell depleted mice the disease process went on in a fashion similar to untreated mice infected with ts1. Thus, this study demonstrates that the disease syndrome induced by ts1 in BALB/c mice is dependent upon the presence of T lymphocytes during the early course of infection, and that presence of B lymphocytes have little or no effect on the disease outcome.