LAMTOR2 Is Dispensable for T Cell Receptor-Mediated Signaling of Thymocytes but Controls Medullary Thymic Epithelial Cells

Human LAMTOR2 deficiency is characterized by severe congenital neutropenia, growth failure, partial albinism, as well as B and T cell deficiencies (Bohn et al., Nat Med 2007). To determine the role of the endosomal adaptor LAMTOR2 in T cell development and homeostasis we used conditional knockout mouse models.

Mx1-Cre-driven knockout of Lamtor2 resulted in reduction of thymus weight and total thymocyte numbers. Immunophenotyping revealed an impaired T cell development characterized by a partial block at the double negative CD4^-CD8^- T cell precursor stage after 7 and 21 days of poly I:C injection that induced deletion of the Lamtor2 gene. Since Mx1-Cre-driven knockout does not allow a discrimination between T cell intrinsic and extrinsic effects, we next generated pre-TCRα-iCre conditional knockout mice. In contrast to Mx1-Cre-Lamtor2^fl/fl mice, mice with T cell-specific knockout of Lamtor2 showed normal frequencies of total thymocytes and T cell progenitor subsets. Furthermore, LAMTOR2-deficient thymocytes exhibited normal TCR signaling (p-ERK, p-LAT, p-LCK, p-PLCγ, Nur77) and internalization of TCRβ upon stimulation with anti-CD3ε +/- anti-CD28, indicating that LAMTOR2 in T cells is dispensable for thymocyte development.

To assess whether T cell developmental defects in Mx1-Cre-Lamtor2^fl/fl mice are caused by a dysfunctional thymic epithelium, we analyzed thymic epithelial cells (TECs) after 4 days of poly I:C injection by flow cytometry and detected a reduced ratio of CD45^-EpCAM⁺UEA-1⁺Ly51^- medullary TECs (mTECs) to CD45^-EpCAM⁺UEA-1^-Ly51⁺ cortical TECs in LAMTOR2-deficient mice. Further studies are underway to determine the role of LAMTOR2 in mTECs.

Taken together, our findings show that LAMTOR2 is not required for TCR-mediated signaling but plays a critical role in controlling mTEC homeostasis.