The effect of cyclosporin on major histocompatibility complex-linked resistance to murine cytomegalovirus

The ability of mice to survive infection with murine cytomegalovirus (MCMV) is known to be influenced by genes of the major histocompatibility complex (MHC). One hypothesis to account for this association is that MHC-linked resistance to MCMV is an ‘immune response’ gene effect, caused by differences in the strength of the MHC-restricted T cell response of mouse strains with different MHC haplotypes. Therefore, removal of T cell responses in mouse strains differing only at the MHC should render them equally susceptible to the virus infection. To test this hypothesis, the immunosuppressive drug cyclosporin (CsA) was used to reduce T cell responses in inbred congenic mouse strains carrying either a resistant or susceptible MHC haplotype. CsA reduced the delayed-type hypersensitivity (DTH) response to MCMV in both resistant and susceptible mouse strains to background levels, equivalent to control uninfected mice. CsA treatment had little effect on the susceptibility of C57BL/10 and B10.BR mice to the virus and the differences in susceptibility between these strains remained. In contrast, CsA increased the susceptibility of the genetically susceptible BALB/c mice (H-2d) by 100-fold and increased the susceptibility of resistant BALB.K mice (H-2k) by 15-fold. Thus the H-2-determined difference in susceptibility between these strains was increased after CsA treatment. The results obtained with congenic strains show that MHC-linked resistance patterns to MCMV are not eliminated by CsA and suggest therefore that T cells are not responsible for this phenomenon. Interestingly, the mean time to death was delayed for CsA-treated BALB/c mice compared with untreated mice given equivalent virus doses. In addition, although CsA prevented DTH responses in both genetically susceptible A/J (H-2a) and resistant CBA (H-2k) mice, CsA treatment markedly increased the susceptibility of A/J mice (32-fold) but had little effect on the susceptibility of CBA mice to the virus

Delayed-type hypersensitivity responses to murine cytomegalovirus in genetically resistant and susceptible strains of mice

The delayed-type hypersensitivity (DTH) response in mice infected with murine cytomegalovirus (MCMV) was measured by ear swelling following a challenge with heat-treated MCMV. DTH was dose-dependent and could be detected as early as 3 days post-infection with peak responses occurring between days 15 and 21 post-infection. The DTH response was found to be specific for MCMV since it could not be elicited by either herpes simplex virus type 1 or influenza A virus in MCMV-primed mice. The elicited DTH response was greater in mice primed with attenuated compared with virulent MCMV. The DTH response was shown to correlate positively with the genetically determined resistance status of mouse strains to this virus. Previous research has shown that resistance to lethal infection with MCMV is controlled by H-2-linked genes since mice having the k haplotype are more resistant than mice having the b or d haplotype of the H-2-complex. Also, non-H-2-linked genes in CBA, C3H, C57BL/10 and probably other strains confer resistance. Resistant strains (C3H [H-2k], CBA [H-2k]) developed greater DTH responses than those of susceptible strains (BALB/c [H-2d], C57BL/10 [H-2b]) inoculated with the same dose of virus. In addition, the genetically resistant mouse strains B10.BR [H-2k] and BALB.K [H-2k] gave a significantly greater DTH response than that of the corresponding congenic strains C57BL/10 [H-2b], BALB/c [H-2d] and BALB.B [H-2b] which are genetically susceptible to the virus. Also, the DTH response of C57BL/10 [H-2b] was significantly higher than that of BALB.B [H-2b] which correlates with their relative genetic resistance to MCMV, indicating the importance of non-H-2-linked genes. Furthermore, in addition to the response of greater magnitude, resistant strains (CBA, C3H, B10.BR) produced DTH responses to MCMV by day 3 compared with day 5 post-infection for susceptible BALB/c mice. These findings indicate that the magnitude and the time of appearance of the DTH response correlates positively with the genetically determined resistance status, although the role of DTH responses in controlling MCMV infections remains to be determined

Antibody responses to murine cytomegalovirus in genetically resistant and susceptible strains of mice

The role of antibodies as mediators of genetically determined resistance to murine cytomegalovirus (MCMV) in mice has not been elucidated. The ability of mice with different MCMV resistance phenotypes to produce an antibody response to MCMV was investigated in order to assess whether the host genotypes that control resistance also influence antibody production. Antibodies to MCMV in the sera of resistant (BALB.K, CBA/CaH, B10.BR) and susceptible [BALB/c, BALB.B, C57BL/10ScSn (B10), B10.D2, B10.A, A/J] mice were determined by ELISA and/or a complement-requiring neutralization assay. IgM antibodies were produced by all strains of mice as early as 3 to 5 days post-infection (p.i.) with maximum titres observed after 10 days p.i. for some strains, whilst IgG antibodies were produced by 5 to 7 days p.i. with maximum titres at 20 days p.i. IgA antibodies were not detected in the sera of MCMV-infected mice. Virulent MCMV induced higher antibody titres than either attenuated or u.v.-inactivated forms of the virus. Although high doses of virulent virus delayed the early production of IgM antibody they did not adversely affect the kinetics of IgG antibody production. High titres of neutralizing antibodies were detected as early as day 3 post-inoculation of virulent virus; when attenuated virus was used in the neutralization assay, this was found to be more easily neutralized than salivary gland-derived virus. Interestingly, although guinea-pig complement greatly enhanced antibody-mediated neutralization of MCMV, mouse complement was also effective at enhancing neutralization. Although genetically determined resistance to MCMV is an early event with the resistant phenotype being demonstrable during the first few days of the infection, there was no evidence that antibodies were responsible for this resistance since neither antibody titres nor the time of first appearance of antibody correlated with resistance status. However, these results do not exclude a more general role for antibody in limiting MCMV infection, especially in immunity to re-infection since passively transferred antibodies from resistant or susceptible mouse strains lowered virus titres in MCMV-infected animals

Cytomegaloviruses: Murine and other nonprimate cytomegaloviruses

The cytomegaloviruses (CMVs) are large, enveloped, double-stranded DNA viruses that belong to subfamily Betaherpesvirinae of the family Herpesviridae. The best described of the nonprimate CMVs is murine cytomegalovirus (MCMV; Murid herpesvirus 1). MCMV is the type species for the genus Muromegalovirus, which also contains rat cytomegalovirus (RCMV; Murid herpesvirus 2). Putative CMVs that have yet to be classified fully include guinea pig cytomegalovirus (GPCMV; Caviid herpesvirus 2), tree shrew herpesvirus (Tupaiid herpesvirus 1), swine cytomegalovirus (suid herpesvirus 2), European ground squirrel cytomegalovirus (sciurid herpesvirus 1), and American ground squirrel cytomegalovirus (sciurid herpesvirus 2). The genomes of MCMV and RCMV have similar genetic structures to those of primate CMVs. Genes in the central region of the genome are conserved among all CMVs, whereas viral species-specific genes are found near the termini. Nonprimate CMVs have been used widely to investigate basic mechanisms of viral persistence, latency, and pathogenesis, as well as to elucidate innate and adaptive immune responses to the virus. CMVs are strictly species specific, and rodent CMVs in their natural hosts have been used widely as experimental models of human CMV (HCMV) infection. MCMV has been used to study a variety of virus-induced diseases, such as hepatitis and myocarditis, and RCMV is used as a model of the vascular diseases associated with HCMV infection. GPCMV crosses the placenta and is used as a model of congenital HCMV infection

Genes of murine cytomegalovirus exist as a number of distinct genotypes

Murine cytomegaloviruses encode a number of genes which modulate polymorphic host immune responses. We suggest that these viral genes should themselves therefore exhibit sequence polymorphism. Additionally, clinical isolates of human cytomegalovirus (HCMV) have been shown to vary extensively from the common laboratory strains. Almost all research conducted on murine cytomegalovirus (MCMV) has used the laboratory strains Smith and K181, which have been extensively passaged in vitro and in vivo since isolation. Using the heteroduplex mobility assay (HMA) to determine levels of sequence variation 11 MCMV genes were examined from 26 isolates of MCMV from wild mice, as well as both laboratory strains. Both the HMA and sequencing of selected genes demonstrated that whilst certain genes (M33, mck-2, m147.5, m152) were highly conserved, others (m04, m06, M44, m138, m144, m145 and m155) contained significant sequence variation. Several of these genes (m06, m144 and m155) exist in wild MCMV strains as one of several distinct genotypes