The Catalytic Activity of the Mitogen-Activated Protein Kinase Extracellular Signal-Regulated Kinase 3 Is Required To Sustain CD4⁺ CD8⁺ Thymocyte Survival

Abstract

Extracellular signal-regulated kinase 3 (ERK3) is an atypical member of the mitogen-activated protein kinase (MAPK) family whose function is largely unknown. Given the central role of MAPKs in T cell development, we hypothesized that ERK3 may regulate thymocyte development. Here we have shown that ERK3 deficiency leads to a 50% reduction in CD4 ؉ CD8 ؉ (DP) thymocyte number. Analysis of hematopoietic chimeras revealed that the reduction in DP thymocytes is intrinsic to hematopoietic cells. We found that early thymic progenitors seed the Erk3 ؊/؊ thymus and can properly differentiate and proliferate to generate DP thymocytes. However, ERK3 deficiency results in a decrease in the DP thymocyte half-life, associated with a higher level of apoptosis. As a consequence, ERK3-deficient DP thymocytes are impaired in their ability to make successful secondary T cell receptor alpha (TCR␣) gene rearrangement. Introduction of an already rearranged TCR transgene restores thymic cell number. We further show that knock-in of a catalytically inactive allele of Erk3 fails to rescue the loss of DP thymocytes. Our results uncover a unique role for ERK3, dependent on its kinase activity, during T cell development and show that this atypical MAPK is essential to sustain DP survival during RAG-mediated rearrangements.

The atypical MAPK ERK3 controls positive selection of thymocytes

“…Previous work from our laboratory has also shown that the thymic epithelium contributes to the decreased generation of SP thymocytes in the absence of ERK3 . However, we do not think that ERK3 deficency in the epithelium is responsible for the defective positive selection phenotype because we could recapitulate the results with in vitro stimulation of thymocytes.…”

“…To bypass this problem, we have introduced already rearranged TCR‐ α and TCR‐ β transgenes into the ERK3‐deficient background. As previously shown, introduction of the OT‐II TCR specific for ovalbumin in the context of the MHC class II molecule I‐A b restores thymic cellularity and DP thymocyte number allowing us to study further steps in T‐cell differentiation (Fig. ).…”

“…a) that are on their way to become DP thymocytes and as such they do not express the TCR (data not shown). We have previously published that ERK3 deficiency reduces DP and CD4SP thymocyte numbers . However, in these experiments, we did not evaluate whether ERK3 influences positive selection of thymocytes.…”

“…Although the results presented above suggested a role for ERK3 during positive selection, it was important to rule out any other possible defect that could lead to a reduction in the generation of SP thymocytes. We have previously shown that ERK3 is necessary for the proper survival of DP thymocytes to allow them to test different TCR‐ α chain rearrangements . This could directly influence positive selection by limiting the chance of a DP thymocyte to express a TCR permissive to positive selection.…”

B Cell Receptor Signaling

“…In case of apoptosis induction in antigen receptor-stimulated B and T cells, inhibition of p38 MAPK activity by SB203580 did not affect apoptosis induction in both cell types (Salmon et al 1997). in the half-life of double-positive thymocytes, associated with a higher level of apoptosis, as well as impaired positive selection (Marquis et al 2014b;Sirois et al 2014). Thus, p38 MAPK signaling can exert agonistic or antagonistic effects on apoptosis induction, which is dependent on the cell type and context.…”