CD4 − CD8 − thymocytes from MRL-lpr/lpr mice exhibit abnormal proportions of αβ- and γδ-TCR + cells and demonstrate defective responsiveness when activated through the TCR

Abstract

MRL-lpr/lpr (lpr) mice develop profound lymphadenopathy resulting from the accumulation of CD4 −CD8 − (double-negative, DN) cells in the peripheral lymphoid organs. Earlier studies from our laboratory demonstrated an increased proportion of DN cells in the thymus of lpr mice with age. Inasmuch as the DN thymocytes constitute a heterogenous population of cells, in the present study, we investigated the TCR phenotype of DN thymocytes and their responsiveness to activation through the TCR. The DN thymocytes of young (1 month of age) lpr mice contained ~65% CD3 + cells of which ~60% were α/gb-TCR + and ~39% were γδ-TCR + as detected by using pan anti-TCR mAbs. In old (4–6 months of age) or young MRL-+/+ mice, similar proportions of CD3 +, αβ- or γδ-TCR + DN thymocytes were detected. Interestingly, however, in old (4–6 months of age) lpr mice, the CD3 + T cells increased to ~86% and the majority of these (~81%) were αβ-TCR + and only ~3% were γδ-TCR +. Also, in old lpr mice, there was a 10-fold increase in the absolute number of αβ-TCR + DN cells in the thymus, whereas, the absolute number of γδ-TCR + DN cells in the thymus did not alter significantly. Furthermore, a majority (~84%) of the old lpr DN thymocytes expressed CD45R, similar to the peripheral DN T cells. In contrast, only a small number (~ 1%) of DN thymocytes from young Ipr or MRL-+/+ mice expressed CD45R. The DN thymocytes from young Ipr or MRL-+/+ mice demonstrated strong and similar proliferative responsiveness to stimulation with PMA + calcium ionophore or PMA + IL-2, or to immobilized mAb directed against the TCRs (CD3, αβ and γδ). In contrast, the DN thymocytes and the DN peripheral T cells from old lpr mice demonstrated marked defect in responding to the above stimuli. The present study suggests that with the onset of lymphadenopathy, the DN cells in the thymus of old lpr mice are increasingly skewed toward the αβ-TCR repertoire, the majority of which express CD45R and respond poorly to mitogenic stimuli or when activated through the TCR. It is suggested that migration of such cells continuously to the periphery may result in severe lymphadenopathy seen in old MRL-lpr/lpr mice.