The lymphocyte-specific protein tyrosine kinase (LCK) is the principal initiator of T cell receptor (TCR) signaling and a critical regulator of T-cell development, activation, and immune tolerance. Its critical expression in T cells positions LCK as both a molecular gatekeeper of adaptive immunity and an attractive therapeutic target. Loss-of-function mutations in LCK result in combined immunodeficiency with profound defects in T-cell differentiation and signaling, whereas hypomorphic variants with residual activity can promote immune dysregulation and autoimmunity. Conversely, aberrant LCK activation is associated with autoimmunity and chronic inflammation, including type 1 diabetes, psoriasis, rheumatoid arthritis, asthma, multiple sclerosis, and graft-versus-host disease. Here, we review the structural and regulatory mechanisms of LCK, its dysfunction in immunodeficiency and autoimmunity, and the current landscape of therapeutic strategies. While first-generation inhibitors targeting the ATP-binding site have shown efficacy in preclinical models, their lack of selectivity highlights the need for innovative approaches directed at non-catalytic domains of LCK. By linking mechanistic understanding with clinical relevance, we position LCK as a precision regulatory node with the potential to transform therapeutic strategies for T cell-driven immune disorders.

Severe combined immunodeficiency (SCID) encompasses rare primary immunodeficiency disorders characterized by deficient T-cell development, which leads to a severely compromised immune system and susceptibility to life-threatening infections. Among SCID subtypes, IL7RA-SCID is caused by mutations in the interleukin 7 receptor alpha chain (IL7RA) and represents a significant subset of patients with limited treatment options. This study investigated the efficacy of a self-inactivating (SIN) alpharetroviral vector (ARV) engineered to deliver a codon-optimized IL7RA cDNA to restore T-cell development in Il7r-knockout mice. We compared the elongation factor 1 alpha short (EFS) promoter and the lymphoid-restricted Lck promoter for their ability to drive IL7RA expression and found that the EFS promoter enabled robust and sustained IL7RA expression that led to the functional rescue of T-lymphopoiesis in vitro and in vivo. Conversely, though effective in vitro, the Lck promoter failed to produce viable T-cell populations in vivo. Our results highlight the potential of using SIN-ARVs as a gene therapy (GT) strategy for treating IL7RA-SCID. Importantly, sustained production of T-lymphocytes was found in both primary and secondary transplant recipient animals with no adverse effects, supporting the safety and feasibility of this approach. Overall, this study provides valuable insights into the development of GT for IL7RA-SCID and underscores the clinical potential of an EFS-driven SIN-ARV to restore IL7RA-deficient immune function.

Mutations affecting T-cell receptor (TCR) signaling typically cause combined immunodeficiency (CID) due to varying degrees of disturbed T-cell homeostasis and differentiation. Here, we describe two cousins with CID due to a novel nonsense mutation in LCK and investigate the effect of this novel nonsense mutation on TCR signaling, T-cell function, and differentiation. Patients underwent clinical, genetic, and immunological investigations. The effect was addressed in primary cells and LCK-deficient T-cell lines after expression of mutated LCK. RESULTS: Both patients primarily presented with infections in early infancy. The LCK mutation led to reduced expression of a truncated LCK protein lacking a substantial part of the kinase domain and two critical regulatory tyrosine residues. T cells were oligoclonal, and especially naïve CD4 and CD8 T-cell counts were reduced, but regulatory and memory including circulating follicular helper T cells were less severely affected. A diagnostic hallmark of this immunodeficiency is the reduced surface expression of CD4. Despite severely impaired TCR signaling mTOR activation was partially preserved in patients' T cells. LCK-deficient T-cell lines reconstituted with mutant LCK corroborated partially preserved signaling. Despite detectable differentiation of memory and effector T cells, their function was severely disturbed. NK cell cytotoxicity was unaffected. Residual TCR signaling in LCK deficiency allows for reduced, but detectable T-cell differentiation, while T-cell function is severely disturbed. Our findings expand the previous report on one single patient on the central role of LCK in human T-cell development and function.