MCL-1 keeps a charming home for thymocytes

In this issue of Blood, Jain et al use a conditional knockout mouse model to demonstrate that MCL-1 expression in thymic epithelial cells (TECs) is important for medullary epithelial cell survival, maintenance of thymic architecture, and support of thymocyte differentiation.¹ 

Crosstalk between thymocytes and TECs plays an important role in thymocyte differentiation.²  Although epithelial cells in the cortex (cTECs) support T progenitor cell proliferation and induce positive selection of thymocytes with diverse, functionally competent T-cell receptor repertoires, epithelial cells in the medulla (mTECs) are essential to induce negative selection and T regulatory cell generation to ensure self-tolerance. Although great efforts have been employed to understand the origins and development of cTECs and mTECs,³  few studies have addressed the maintenance of TECs.

Jain et al investigated the function of Bcl-2 family proteins in TEC homeostasis. The authors identified 3 prosurvival Bcl-2 family genes, Bcl2, Bcl2l1, and Mcl1, that are expressed in cTECs and mTECs. To address if the prosurvival proteins encoded by these genes are important for TEC homeostasis and the maintenance of thymic function, Jain et al specifically deleted these genes only in TECs by using Foxn1^Cre or K5^Cre. TEC-specific deletion of Bcl2 or Bcl2l1 did not reveal abnormalities in the thymus. In striking contrast, TEC-specific deletion of Mcl1 (Mcl1^cKO) induced marked thymic hypoplasia (see figure).¹  Jain et al examined the impact of the loss of MCL-1 on cTECs and mTECs. The authors reported a severe deficiency in mTEC numbers with a complete absence of thymic medulla by 2 months of age. Surprisingly, MCL-1 deficiency did not affect the overall number of cTECs. These results reveal for the first time a novel role for MCL-1 in TEC homeostasis.

One intriguing finding in this study is that even though all 3 prosurvival Bcl-2 family members (BCL-2, BCL-XL, and MCL-1) were expressed in mTECs, MCL-1 deficiency caused apoptosis only in mTECs, not in cTECs. Why mTECs are susceptible to apoptosis awaits further investigation. mTECs promiscuously transcribe tissue-restricted antigens (TRAs), the expression of which is otherwise restricted to differentiated organs. This process is controlled by the autoimmune regulator (AIRE), which is found to be associated with proteins implicated in DNA damage responses.⁴  One possibility is that transcription of otherwise silenced genes in mTECs results in the DNA damage responses, which in turn activate the cell death effector molecule BAK.⁵  However, the effector function of BAK can be neutralized by MCL-1 expression in mTECs. In this regard, Jain et al showed that deletion of BAK in TECs completely rescued the thymic hypoplasia in Mcl1^cKO mice.

Another exciting question that remains to be explored is the biological significance of mTECs being more susceptible to apoptosis than cTECs. Gray et al reported earlier that the most mature AIRE⁺ mTEC population expressing the most TRAs had a high turnover rate followed by apoptosis.⁶  Although it is known that each TRA is only expressed by a minor fraction (1% to 3%) of mTECs at any given time,⁷  it is possible that the continuous rapid cycling of mTECs followed by apoptosis could be a mechanism to maximize the exposure of a broad spectrum of TRAs to thymic dendritic cells. Cross-presentation of mTEC-derived TRAs by dendritic cells is crucial to establish self-tolerance. It may be informative to investigate if TRA-mediated clonal deletion is impaired in mice with BAX and BAK double deficiency in TECs.

Jain et al discovered that epidermal growth factor (EGF) is the key stimulator to induce MCL-1 expression via the MAPK/ERK kinase pathway in TECs. EGF receptor signaling also activates STAT3. Recent studies show that STAT3 deletion in TECs results in significant reduction in the number of mTECs.^8,9  It is possible that EGF also induces MCL-1 expression in TECs through STAT3 activation, but the maximal induction of MCL-1 depends on both the MAPK kinase and STAT3 pathways. Whether MCL-1 expression is responsible for the impaired maintenance of mTECs in Stat3^cKO mice and whether BAK deletion in TECs can fully restore the thymic medulla in Stat3^cKO mice remain to be determined. Given that EGF is the major cytokine to induce MCL-1 expression in TECs, it may be important to assess if EGF is limiting in the thymus and, if so, to what extent EGF contributes to the rapid cycling and apoptosis in mTECs under physiological conditions.

Jain et al examined thymic function after MCL-1 deletion in TECs. The authors observed a great deficit in all thymocyte subsets, including DN, DP, CD4SP, and CD8SP thymocytes in Mcl1^cKO mice. Importantly, the authors also showed that the reduction in DN thymocytes occurred in all 4 subsets (DN1, DN2, DN3, and DN4), indicating that the disrupted thymic microenvironment affected thymocyte differentiation from their early progenitors. Although the total cell numbers of cTECs remained unchanged in the Mcl1^cKO thymus, their functions were impaired.¹  Past work shows that lack of thymic medulla by itself, for example in RelB^KO mice, does not result in thymic hypoplasia.¹⁰  One difference between Mcl1^cKO and RelB^KO mice is that the Mcl1^cKO thymus has impaired cTEC functions, whereas the RelB^KO thymus maintains normal cTEC functions. These findings suggest that rather than the lack of a thymic medulla, the impairment in cTECs is responsible for thymic hypoplasia in Mcl1^cKO mice. In addition, thymi of Mcl1^cKO mice were overpopulated with thymic fibroblasts. Taken together, the present study demonstrates an important role for MCL-1 in maintaining the thymic architecture that supports thymic function.

On the basis of the findings from the present study, developing strategies to control the balance of MCL-1 and BAK could be beneficial for future therapies aiming to restore or improve thymic function in various clinical situations, including immune-cell reconstitution after chemotherapy. However, when designing therapies to improve TEC proliferation and survival, it is important to keep in mind that dying mTECs are an important source of antigens for thymic dendritic cells that are involved in self-tolerance.