Characterization of a Mutant Mouse Strain with Altered Peripheral CD4 T Cell Homeostasis
The mechanisms regulating peripheral CD4+ T cell homeostasis are not completely understood. Previous research at Davidson College has identified a mutant strain of mice with an inverted CD4/CD8 ratio in peripheral lymphoid organs and blood. Furthermore, the mutant strain shows an increased incidence of encephalomyelitis, right-eye blindness, and skin lesions in the absence of infection, indicating a possible autoimmune condition. The purpose of this study is to explore the mechanisms behind the altered T cell ratio, homeostasis, and potential autoimmunity in these mice. Cell counts indicate that the mutant strain exhibits a highly reduced frequency of CD4 T cells while the CD8 T cells remain largely unaffected when compared to control strains. First, we examined whether the CD4 cells in the mutant mouse are more susceptible to apoptosis. AnnexinV/7AAD staining found no differences between mutant and control mice in the frequencies of AnnexinV+CD4+ or AnnexinV+ CD8+ cells in the presence or absence of TCR stimulation. Then, we examined the role of various factors involved in peripheral homeostasis. We investigated IL-7 signaling in peripheral T cells by culturing lymphocytes overnight in IL-7 deficient medium and analyzing the dynamics of IL-7R expression using flow cytometry. The data show that IL-7R is upregulated normally in the mutant strain when compared to a control of the same genetic background, indicating that IL-7 signaling is not impaired. Finally, we investigated the functionality of mature T cells from the mutant strain. In vitro TCR stimu
lation of CFSE-labeled T lymphocytes with either alloreactive APCs or anti-TCR-beta antibody demonstrated that CD4+ and CD8+ cells from the mutant activated and proliferated in numbers comparable to the control strain. In order to evaluate T cell homeostatic proliferation, we performed an in vivo BrdU labeling experiment, followed by flow cytometry. Results indicate that more BrdU incorporation occurred in CD8 cell than CD4 cells in the mutant, but that both classes of T cell proliferated at roughly double the rate of the control strain. This finding could partially explain the skewing of the CD4/CD8 ratio in favor of the CD8 cells. We think that this may also be consistent with the putative autoimmune condition in the mutant strain. If CD4 cells are infiltrating non-lymphoid tissues, more rapid proliferation of both cell types could be a compensatory mechanism occurring to fill the niche vacated by the autoreactive CD4 cells. This would explain both the decreased frequency of CD4 cells and the increased rate of proliferation.
Carico, ZM; Spasova, DS; Sarafova, SD
Midwinter Conference of Immunologists
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