Systemic MCMV Infection in Mice without or with MAIDS

Systemic MCMV infection moderately stimulates splenic SOCS1, SOCS3, and

SOCS-inducing cytokines in C57BL/6 mice without MAIDS, but not with MAIDS.

Intraperitoneally-injected MCMV at sub-lethal doses causes a self-limited systemic infection

which disseminates through the blood to replicate in various organs, including the spleen [245,

370, 371]. This results in mild, transient splenomegaly that involves relatively proportional

expansion of leukocytes, and acute replication that peaks between 3–12 days post-infection [370,

383]. Herein we found that systemic MCMV infection in the absence of retinal disease or

immune suppression caused mild, transient stimulation of splenic SOCS1 and SOCS3 protein

that was not prolonged and did not reach the amplitude of stimulation found during intraocular

MCMV infection in retinitis-susceptible MAIDS-10 mice.

In the spleens of immunologically normal mice without MAIDS, moderate stimulation of

SOCS1 and SOCS3 occurred with peak IFN-γ and IL-6 mRNA expression. We therefore cannot eliminate the possibility that splenic SOCS1 and/or SOCS3 expression during systemic MCMV

infection could be an indirect immunologic consequence of MCMV stimulating IFN-γ, IL-6, or other cytokines. Because systemic MCMV infection failed to stimulate splenic IFN-α and IFN-β mRNA transcripts at any time point investigated, it is unlikely that these type I IFNs are involved

in SOCS1 and/or SOCS3 stimulation under these conditions, although we cannot rule out the

possibility that these may be stimulated at time points not observed in this study.

The significant down-regulation of IFN-α and IFN-β mRNA expression at day 4 post- infection, which followed peak SOCS1 mRNA production, is consistent with previous findings

in vivo [383] and in vitro in mouse fibroblast and macrophage cell lines [375]. Because SOCS1

type I IFN, including NF-κB [321], it is not unreasonable to hypothesize that the modest induction of SOCS1 found prior to type I IFN down-regulation may contribute to the down-

regulation of type I IFN during systemic MCMV infection. Although MCMV also employs other

virologic mechanisms that down-regulate type I IFN in a transcriptional or functional capacity

(reviewed in [339]), SOCS1 and/or SOCS3 may contribute to the arsenal that MCMV utilizes to

counteract these antiviral cytokines.

During the progression of MAIDS, systemic MCMV-related stimulation of SOCS1,

SOCS3, and IL-6 in the spleen was abolished as early as MAIDS-4. Although splenic IFN-γ mRNA expression demonstrated intermediate stimulation at MAIDS-4, this effect was

insufficient to stimulate SOCS1 or SOCS3 expression at that time point, and it was completely

eliminated by MAIDS-10, suggesting a progressive decline in the ability of systemic MCMV to

induce SOCS during the development of MAIDS. The phenomenon or cell type(s) responsible

for modulation of splenic SOCS1, SOCS3, and/or SOCS-inducing cytokines during acute,

systemic MCMV infection in immunologically normal mice, therefore, is progressively either

abrogated or masked by the effects of MAIDS. It is possible that the MAIDS-inducing retrovirus

mixture itself may interfere with the ability of MCMV to stimulate these host proteins. However

possible, this is unlikely because of the robust stimulation of ocular SOCS1 and SOCS3 in the

MCMV-infected eyes of MAIDS-10 mice. In addition, when compared with whole spleens from

age-matched immunologically normal (healthy) control mice, mid-stage and late-stage MAIDS

do not affect splenic SOCS1 mRNA and progressively increases SOCS3 mRNA in whole splenic

cells [350]. Therefore, if the retrovirus mixture does interfere with the ability of MCMV to

stimulate SOCS1 and/or SOCS3, it does so in a highly tissue-specific and/or cell-type-specific

splenic cells [235], including B and T lymphocytes [230, 231, 236-239], NK cells [240],

neutrophils [241], and macrophages [236, 239, 242], it is possible that any of these cell types that

might normally stimulate SOCS1 and/or SOCS3 expression fail to do so when rendered

dysfunctional by MAIDS progression.

Alternatively, because MAIDS causes severe splenomegaly by aberrant proliferation of

many splenic cell types, the primary splenic cells responsible for SOCS1 and/or SOCS3

expression during systemic MCMV infection without MAIDS could be underrepresented during

MAIDS progression due to overpopulation of non-SOCS-expressing cells in the whole spleens of

mice with MAIDS. This would further suggest that the splenic cell types undergoing aberrant

proliferation as early as MAIDS-4 are not responsible for SOCS1 and/or SOCS3 expression

during MCMV infection, and would therefore rule out B cells [230, 236, 237] as well as CD4+

and CD8+ T cells [231, 238, 239]. Reduced Mac1+ (CD11b+

Systemic MCMV infection fails to stimulate ocular SOCS1, SOCS3, or SOCS-

inducing cytokines in C57BL/6 mice during the progression of MAIDS. Although systemic

MCMV infection in the absence of retrovirus-induced immune suppression induced moderate

amounts of splenic SOCS1 and SOCS3 proteins, this MCMV-related up-regulation of splenic or

ocular SOCS1 or SOCS3 did not occur during systemic MCMV infection of MAIDS-4 or

MAIDS-10 mice in the absence of retinitis. The amplitude of SOCS1 and SOCS3 production

during MCMV infection is therefore correlated with severity of MAIDS-related MCMV retinitis.

Taken together, these data suggest that, similar to other viruses, MCMV may also induce and ) macrophage population

percentages and activation frequencies have been reported during MAIDS-4 [229, 243],

positioning these cell types as potential candidates for SOCS1 and/or SOCS3 producers in whole

exploit SOCS protein expression [326-328], but perhaps only under specific conditions that are

related to ocular disease.