Exacerbated metastatic disease in a mouse mammary tumor model following latent gammaherpesvirus infection

Prevention of Tumor Formation by Latent Gammaherpesvirus Infection

Recent reports suggested that chronic herpesvirus infection, as a constituent of the so-called virome, may not only exert harmful effects but may also be beneficial to the host, for example mediating increased resistance to secondary infections or to tumors. To further challenge this concept, specifically regarding increased resistance to tumors, we infected chimeric HLA-DR4-H2-E (DR4) mice, a mouse strain which spontaneously develops hematological tumors, with the rodent herpesvirus murine gammaherpesvirus 68 (MHV-68). Using this model, we observed that infection with wildtype MHV-68 completely prevented tumor formation. This happened, however, at the cost of hyposplenism. In contrast to wildtype infection, infection with a latency-deficient mutant of MHV-68 neither prevented tumor formation nor induced hyposplenism. The underlying mechanisms are not known but might be related to an infection-mediated priming of the immune response, resulting in the suppression of a tumor promoting endogenous retrovirus. Thus, under certain circumstances, chronic herpesvirus infection may prevent the development of tumors.

Alpha beta-crystallin expression and presentation following infection with murine gammaherpesvirus 68

Alpha beta-crystallin (CRYAB) is a small heat shock protein that can function as a molecular chaperone and has protective effects for cells undergoing a variety of stressors. Surprisingly, CRYAB has been identified as one of the dominant autoantigens in multiple sclerosis. It has been suggested that autoimmune mediated destruction of this small heat shock protein may limit its protective effects, thereby exacerbating inflammation and cellular damage during multiple sclerosis. It is not altogether clear how autoimmunity against CRYAB might develop, or whether there are environmental factors which might facilitate the presentation of this autoantigen to CD4+ T lymphocytes. In the present study, we utilized an animal model of an Epstein Barr Virus (EBV)-like infection, murine gammaherpesvirus 68 (HV-68), to question whether such a virus could modulate the expression of CRYAB by antigen presenting cells. Following exposure to HV-68 and several other stimuli, in vitro secretion of CRYAB and subsequent intracellular accumulation were observed in cultured macrophages and dendritic cells. Following infection of mice with this virus, it was possible to track CRYAB expression in the spleen and in antigen presenting cell subpopulations, as well as its secretion into the blood. Mice immunized with human CRYAB mounted a significant immune response against this heat shock protein. Further, dendritic cells that were exposed to HV-68 could stimulate CD4+ T cells from CRYAB immunized mice to secrete interferon gamma. Taken together these studies are consistent with the notion of a gammaherpesvirus-induced CRYAB response in professional antigen presenting cells in this mouse model.

An expanded myeloid derived suppressor cell population does not play a role in gammaherpesvirus-exacerbated breast cancer metastases

Background

Mice latently infected with murine gammaherpesvirus 68 (HV-68) and transplanted with 4 T1 breast cancer cells developed exacerbated metastatic lesions when compared to controls. The mechanisms responsible for this viral-exacerbated disease were not clear. The ability of HV-68 infection to induce S100A8 and S100A9 production and to expand a population of CD11b+Gr-1+ cells suggested that increased numbers, or activity, of viral-expanded myeloid derived suppressor cells (MDSCs) might contribute to HV-68-associated metastatic breast cancer in this model. We questioned whether mock or HV-68 infected mice with significant breast cancer might have differences in the number and/or activity of MDSCs.

Methods

Myeloid-derived macrophages and dendritic cells were isolated from normal mice and cultured in vitro with HV-68 to assess S100A8 and S100A9 mRNA and protein expression. In vivo studies were performed using groups of mice that were mock treated or infected with HV-68. After viral latency was established, 4 T1 breast cancer cells were transplanted in mice. When primary breast tumors were present mice were euthanized and cells isolated for phenotyping of myeloid cell populations using FACS, and for ex vivo analysis of suppressor activity. Serum from these animals was also collected to quantify S100A8 and S100A9 levels.

Results

In vitro studies demonstrated that direct exposure of myeloid cells to HV-68 did not induce increased expression of S100A8 or S100A9 mRNAs or secreted protein. HV-68 infected mice with metastatic breast cancer disease had no increases in S100A8/A9 levels and no significant increases in the numbers or activation of CD11b+Gr-1+MDSCs when compared to mock treated mice with breast cancer.

Conclusions

Together these studies are consistent with the notion that expanded myeloid derived suppressor cells do not play a role in gammaherpesvirus-exacerbated breast cancer metastases. The mechanisms responsible for HV-68 induced exacerbation of metastatic breast cancer remain unclear.