HIP-55 Is Important for T-Cell Proliferation, Cytokine Production, and Immune Responses

The activation of T lymphocytes plays a central role in the regulation of immune responses. An optimal T-cell response requires signals delivered from the T-cell receptor (TCR) and coreceptors. Engagement of the TCR triggers the activation of Src family protein tyrosine kinases Lck and Fyn, which subsequently phosphorylate the immunoreceptor tyrosine-based activation motifs in the CD3ζ tail. The phosphorylated tyrosine-based activation motifs recruit ZAP-70 through the Src homology 2 (SH2) domain of ZAP-70. The recruitment of ZAP-70 to the TCR complex leads to the activation of ZAP-70 by Lck. Upon activation, ZAP-70 plays a major role in propagating the TCR signals to downstream molecules, including LAT, phospholipase Cγ1 (PLCγ1), and SLP-76. LAT is localized in specific plasma membrane compartments known as glycolipid-enriched microdomains and serves as a docking protein for other signaling molecules, including SLP-76 and Grb2. The LAT-associated adaptors then recruit various signaling molecules to the glycolipid-enriched microdomains and promote multiple downstream signaling events, such as mitogen-activated protein kinase activation, the release of intracellular Ca2+, and increased interleukin-2 production (1, 2, 16, 19, 21, 28). HIP-55 (hematopoietic progenitor kinase 1 [HPK1]-interacting protein of 55 kDa; also called SH3P7 and mAbp1) is a novel SH3 domain-containing protein (11). HIP-55 contains an actin-binding domain at its N terminus and an SH3 domain at its C terminus (11), and it binds to filamentous actin and colocalizes with actin filaments (17, 20). HIP-55 also interacts with dynamin and Cdc42, which is important for receptor-mediated endocytosis and Golgi transport (13, 18, 27). HIP-55 is cleaved during apoptosis (6). HIP-55 interacts with HPK1 (11), a serine/threonine protein kinase involved in TCR signaling (15, 23, 24). Upon TCR stimulation, HIP-55 translocates to glycolipid-enriched microdomains (14, 22) and binds to phosphorylated ZAP-70 (14). In HIP-55-knockdown Jurkat T cells established by RNA interference, the signaling events induced by TCR stimulation, such as HPK1 and JNK activation, are defective (14), suggesting that HIP-55 may be an important regulator in TCR signaling. To study the in vivo functions of HIP-55 in TCR signaling and the immune system, we generated and characterized HIP-55 knockout mice. We found that HIP-55 knockout mice showed decreased body weight and higher occurrence of death within the first 4 weeks after birth. The cellularity of lymphoid organs and the development of T cells in HIP-55 knockout mice were normal. However, HIP-55 knockout T cells were less responsive to TCR stimulation, as manifested here by reduced T-cell proliferation, lower cytokine production, and decreased up-regulation of activation markers. Additionally, antigen-specific immune responses were also reduced in HIP-55 knockout mice. The signaling events including LAT/PLCγ1 phosphorylation and HPK1/JNK activation induced by TCR stimulation were partially decreased in HIP-55 knockout T cells, which may be responsible for the defects we observed in the immune system. These findings provide evidence that HIP-55 is an important adaptor protein that regulates T-cell activation and immune responses.