Increased numbers of thymic and peripheral CD4⁺ CD25⁺Foxp3⁺ cells in the absence of CD5 signaling

Abstract

It has been suggested that high affinity/avidity interactions are required for the thymic selection of Treg. Here, we investigated the role of CD5, a negative regulator of TCR signaling, in the selection and function of ''naturally occurring'' CD4 1 CD25 1 Treg (nTreg). Analysis of CD5 À/À mice showed a significant increase in the percentage and absolute numbers of CD4 1 CD25 1 Foxp3 1 thymocytes and peripheral T lymphocytes, compared with BALB/c mice. Thymi from CD5 À/À mice showed reduced cellularity due to increased apoptosis, which preferentially affected naïve T cells. To characterize nTreg selection at the molecular level we investigated the phosphorylation of Erk, c-Cbl, PI3K and Akt. CD5 À/À nTreg showed increased basal levels of p-Erk compared with wild-type nTreg. Interestingly, in response to CD3 plus CD28 costimulation, CD5 À/À naïve T cells but not CD5 À/À nTreg showed lower levels of p-Akt. Finally, CD5 À/À nTreg were thymus-derived and fully functional. We conclude that the enrichment of nTreg observed in the absence of CD5 signaling is due to de novo generation of nTreg and selective reduction of CD4 1 CD25 À naïve thymocytes. Furthermore, we provide new evidence supporting a potential role of CD5 in thymocyte survival, through a mechanism that may involve the phosphorylation of Akt.Key words: CD5 . Development . Treg . Signaling . Thymocyte IntroductionThe maintenance of immunological self-tolerance is a fundamental property of the immune system besides the clearance of pathogenic agents. Central and peripheral mechanisms exist to ensure the elimination or inactivation of potentially harmful autoreactive T cells. Negative selection in the thymus induces the deletion of most self-reactive T-cell clones during T-cell development [1]. However, some autoreactive T cells escape from clonal deletion and need to be controlled by peripheral mechanisms including peripheral T-cell deletion by activation-induced cell death [2], T-cell anergy [3] and T-cell-mediated suppression [4].Recently, non-deletional mechanisms of central tolerance have been re-evaluated, such as the development of thymus-derived Treg [5]. These cells are currently known as ''naturally occurring'' CD4 1 CD25 1 Treg (nTreg), which represent a small population (5-10%) of peripheral CD4 1 T cells and are characterized by high levels of surface CD25 and by the expression of Foxp3, a transcription factor, identified as a key regulator of nTreg development and function [6]. In addition, nTreg also express activation markers such 2233as CTLA-4, glucocorticoid-induced tumor necrosis factor receptor (GITR), CD62L, OX40 and CD103 [7,8]. nTreg were first studied for their role in maintaining self-tolerance [9], preventing autoimmune diseases [10] and the development of harmful immunopathological responses [11]. However, nTreg can also participate in modulating adaptive immunity to viruses, bacteria and parasites affecting favorably or detrimentally the outcome of immune responses to infections [12,13].The mechanisms of suppression used by Treg...

Functional requirement of tyrosine residue 429 within CD5 cytoplasmic domain for regulation of T cell activation and survival

“…Unexpectedly, individual mutations of the cytoplasmic tyrosines of CD5 did not result in an enhanced activation of Caspase 3 ( Fig. 3C and D), in contrast to the result with Annexin V staining and to our previous report in CD5 À/À mice [17]. This could be explained by differences in sensitivity of apoptosis detection between active Caspase 3 and Annexin V staining or might indicate different detection kinetics.…”

“…Previous work by our group demonstrated that thymocytes from CD5 À/À mice display enhanced apoptosis, as demonstrated by Annexin V/7-AAD staining, which correlated with an increased percentage of thymocytes expressing the active form of Caspase 3 [17]. Therefore, we analyzed activation of Caspase 3 in the EL4 clones expressing the different hCD5 mutants (Fig.…”

“…Our group and others have previously described a possible relationship between CD5 and the PI3K/Akt pathway [6,17]. Moreover, we have recently reported that deletion of the carboxyterminal region of CD5 is required for c-Cbl association and Vav degradation, which would presumably associate with CD5 through the recruitment of PI3K to the Y463 [19].…”

“…Moreover, utilization of a CD5DCK2BD deficient mouse showed that this domain plays a central role in peripheral T cell survival, as deletion of this region resulted in enhanced apoptosis in response to CD3-crosslinking, which correlated with a reduction of Akt and GSK3b phosphorylation [14e16]. Recently, our group has demonstrated that deletion of CD5 also decreases survival of developing thymocytes, which show impaired Akt phosphorylation in response to CD3 plus CD28 crosslinking [17]. The molecular mechanisms by which CD5 affects the PI3K/Akt pathway in thymocytes remain unresolved, despite some data argue in favor of the carboxy-terminal region playing a role in PI3K activation [18,19].…”

Exploiting scavenger receptors in cancer immunotherapy: Lessons from CD5 and SR‐B1

“…CD5 expression is notably higher in T-cell subpopulations with regulatory or suppressive functions, i.e. in regulatory T (Treg) cells [41] suggesting that it may regulate their generation and/or function [42][43][44]. Similarly, CD5 is also highly expressed by regulatory B (Breg) cells [45], in which CD5 expression is required for autocrine IL-10 production.…”