Retrovirus-specificity of regulatory T cells is neither present nor required in preventing retrovirus-induced bone marrow immune pathology.

Chronic viral infections of the hematopoietic system are associated with bone marrow dysfunction, to which both virus-mediated and immune-mediated effects may contribute. Using unresolving noncytopathic Friend virus (FV) infection in mice, we showed that unregulated CD4(+) T cell response to FV caused IFN-gamma-mediated bone marrow pathology and anemia. Importantly, bone marrow pathology was triggered by relative insufficiency in regulatory T (Treg) cells and was prevented by added Treg cells, which suppressed the local IFN-gamma production by FV-specific CD4(+) T cells. We further showed that the T cell receptor (TCR) repertoire of transgenic Treg cells expressing the beta chain of an FV-specific TCR was virtually devoid of FV-specific clones. Moreover, anemia induction by virus-specific CD4(+) T cells was efficiently suppressed by virus-nonspecific Treg cells. Thus, sufficient numbers of polyclonal Treg cells may provide substantial protection against bone marrow pathology in chronic viral infections.

Functional impairment of CD8(+) T cells by regulatory T cells during persistent retroviral infection.

The establishment of viral persistence generally requires evasion of the host CD8(+) T cell response. Here we describe a form of evasion wherein the CD8(+) T cells are fully capable of recognizing their cognate antigen but their effector functions are suppressed by regulatory T cells. Virus-specific CD8(+) T cells adoptively transferred into mice persistently infected with Friend virus proliferated and appeared activated, but failed to produce IFNgamma or reduce virus loads. Cotransfer experiments revealed that a subpopulation of CD4(+) T cells from persistently infected mice suppressed IFNgamma production by the CD8(+) T cells. Treatment of persistently infected mice with anti-GITR antibody to ameliorate suppression by regulatory T cells significantly improved IFNgamma production by transferred CD8(+) T cells and allowed a significant reduction in viral loads. The results indicate that CD4(+) regulatory T cells contribute to viral persistence and demonstrate an immunotherapy for treating chronic retroviral infections.

Effective anti-tumor adoptive immunotherapy: utilization of exogenous IL-2-independent cytotoxic T lymphocyte clones.

To attain one of the final goals for cancer immunotherapy, cytotoxic T lymphocyte (CTL) clones were selected on the basis of exogenous IL-2 independence after limiting dilution culture from mixed lymphocyte tumor cell culture cells of FBL-3 tumor-immune spleens. About 10% of the clones could be propagated up to >5 times by weekly passages in the presence of splenic feeder and stimulating tumor cells. Two of the representative FBL-3-specific CTL clones that were able to undergo the fifth passage were expanded in large numbers for adoptive transfer by two rounds of a weekly passage with medium containing IL-2. FBL-3-specific CTL clones thus obtained showed a strong ability to eliminate the established tumors when transferred into tumor-bearing nude mice. In addition, the cells were recovered from the mouse spleen even 8 months after the transfer. The most striking differences between the CTL clones used in this experiment and those maintained conventionally in the presence of IL-2 were the abilities to produce IL-2 by themselves and the high expression level of the integrin molecule, VLA-4, that disappeared when cultured completely in the continuous presence of IL-2 in vitro during 12 weeks. In addition, concomitant with the disappearance of exogenous IL-2 independence and VLA-4 expression, the CTL clones lost their capacity to eradicate the tumor in vivo. Thus, the higher capacity of CTL clones to produce IL-2 on their own seemed to be correlated with the in vivo efficacy for tumor eradication and the long-term maintenance of their physiological profiles typical of memory T cells.