Protein kinase C-θ (PKC-θ) is required for activation of the transcription factor NF-κB induced by signaling via the T cell antigen receptor (TCR); however, the direct activator of PKC-θ is unknown. We report that the kinase GLK (MAP4K3) directly activated PKC-θ during TCR signaling. TCR signaling activated GLK by inducing its direct interaction with the upstream adaptor SLP-76. GLK-deficient mice had impaired immune responses and were resistant to experimental autoimmune encephalomyelitis. Consistent with that, people with systemic lupus erythematosus had considerable enhanced GLK expression and activation of PKC-θ and the kinase IKK in T cells, and the frequency of GLK-overexpressing T cells was directly correlated with disease severity. Thus, GLK is a direct activator of PKC-θ, and activation of GLK-PKC-θ-IKK could be used as new diagnostic biomarkers and therapeutic targets for systemic lupus erythematosus.

JNK pathway-associated phosphatase (JKAP, also known as DUSP22 or JSP-1) is a JNK activator. The in vivo role of JKAP in immune regulation remains unclear. Here we report that JKAP directly inactivates Lck by dephosphorylating tyrosine-394 residue during T-cell receptor (TCR) signalling. JKAP-knockout T cells display enhanced cell proliferation and cytokine production. JKAP-knockout mice show enhanced T-cell-mediated immune responses and are more susceptible to experimental autoimmune encephalomyelitis (EAE). In addition, the recipient mice that are adoptively transferred with JKAP-knockout T cells show exacerbated EAE symptoms. Aged JKAP-knockout mice spontaneously develop inflammation and autoimmunity. Thus, our results indicate that JKAP is an important phosphatase that inactivates Lck in the TCR signalling turn-off stage, leading to suppression of T-cell-mediated immunity and autoimmunity.

Proinflammatory cytokines play important roles in insulin resistance. Here we report that mice with a T-cell-specific conditional knockout of HGK (T-HGK cKO) develop systemic inflammation and insulin resistance. This condition is ameliorated by either IL-6 or IL-17 neutralization. HGK directly phosphorylates TRAF2, leading to its lysosomal degradation and subsequent inhibition of IL-6 production. IL-6-overproducing HGK-deficient T cells accumulate in adipose tissue and further differentiate into IL-6/IL-17 double-positive cells. Moreover, CCL20 neutralization or CCR6 deficiency reduces the Th17 population or insulin resistance in T-HGK cKO mice. In addition, leptin receptor deficiency in T cells inhibits Th17 differentiation and improves the insulin sensitivity in T-HGK cKO mice, which suggests that leptin cooperates with IL-6 to promote Th17 differentiation. Thus, HGK deficiency induces TRAF2/IL-6 upregulation, leading to IL-6/leptin-induced Th17 differentiation in adipose tissue and subsequent insulin resistance. These findings provide insight into the reciprocal regulation between the immune system and the metabolism.

Protein kinase C (PKC)-θ is a serine/threonine kinase belonging to the calcium-independent novel PKC subfamily; its expression is restricted to certain tissues and cell types, including T cells. The signals delivered from T cell receptor (TCR) and CD28 costimulatory molecules trigger PKC-θ catalytic activation and membrane translocation to the immunological synapse, leading to activation of NF-κB, AP-1, and NF-AT. These transcription factors are important for T cell survival, activation, and differentiation. Phosphorylation of PKC-θ at multiple Ser/Thr/Tyr residues is induced in T cells during TCR signaling. Some phosphorylation sites play critical roles in the regulation of PKC-θ function and downstream signaling. The regulation mechanisms for PKC-θ phosphorylation sites are now being revealed. In this review, we discuss the current understanding of the regulation of PKC-θ function by phosphorylation during TCR signaling.

Background: HPK1 is a hematopoiesis-specific Ste20-like serine/threonine kinase that suppresses immune responses and autoimmunity. Results: HPK1 knock-out B cells show loss of Thr-152 phosphorylation, 14-3-3 binding, and Lys-37/38/42-ubiquitination of BLNK. Conclusion: HPK1 attenuates BCR signaling via inducing phosphorylation and ubiquitination of BLNK.Significance: This is the first report of BLNK ubiquitination that is initiated by HPK1-induced phosphorylation.