Phenotypic, Functional, and Developmental Characterization of a Mouse Model for Virtual Memory CD8+ T Cells Open Access
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Virtual memory (VM) cells exist in the steady-state in unimmunized mice as memory phenotype, antigen-specific CD8+ T cells. VM cells display phenotypic and functional similarity to antigen-driven memory cells, however they exist at vanishingly low frequencies making them challenging to isolate, characterize, and study functionally. Our lab generated a mouse model of CD8+ VM cells that allows us to characterize the phenotype, function and developmental requirements of this normally rare cell population. The mouse model, called very low avidity (VLO), harbors CD8+ T cells that express a very low avidity transgenic TCR specific for an epitope of Tyrosinase Related Protein-2 (TRP-2). TRP-2 is an endogenous protein in C57BL/6 mice, and is a tumor antigen expressed in murine melanoma. Phenotypic characterization of VLO cells by flow cytometry shows that their memory signature, CD44hi CD49dlo Eomesoderminhi T-bethi, is consistent with previous reports of VM T cells that originate from a lymphopenia-induced pathway. We hypothesize that VLO cells develop in a unique manner resulting in their acquisition of a memory-like phenotype and innate-like function and may serve as model for non-antigen-induced memory cells. VLO mice exist in a state of severe lymphopenia. When lymphopenic conditions are relieved the frequency of VLO cells expressing a VM phenotype lessens, indicating that cell extrinsic factors are involved in directing acquisition of the VM phenotype. VLO cells proliferate weakly and produce low levels of interferon gamma (IFNγ) in response to antigen stimulation, but respond dramatically to pro-inflammatory cytokines by proliferating robustly and expressing high levels of IFNγ. These responses are consistent with what has been reported for VM cells. In vivo examination of the response of VLO cells to a lymphopenic environment revealed that VLO cells proliferate rapidly, which is consistent with a VM phenotype. When VLO cells were used in an adoptive transfer model for treatment of B16 melanoma they did not control tumor any better than a control group statistically, but further studies are warranted. To gain an understanding of the role of antigen in the development of the memory phenotype of VLO cells, our lab generated a TRP-2 knockout mouse. When VLO cells developed in the absence of their cognate antigen they retained their memory phenotype, but also had increased cell numbers in the periphery suggesting that cognate antigen plays a role in deletion of VLO cells. Using mouse models, we demonstrated that VLO cells are able to develop in the absence of IL-15 or the absence of IL-7. Our data suggest that IL-7 plays a role in the homeostatic maintenance and/or survival of these cells. Taken all together these data support the idea that VLO cells behave consistently with what has been reported about VM cells. We suggest the VLO mouse is a useful model to study the behavior and development of VM cells.