Rajcula, Matthew

Abstract Many tumors exhibit increased incorporation of sialic acids into cell-surface glycans, which impact the tumor microenvironment. Sialic acid immunoglobulin-like lectins (Siglec) are receptors that recognize sialic acids and modulate immune responses, including responses to tumors. However, the roles of individual sialyltransferases in tumorigenesis and tumor growth are not well understood. Here, we examined the sialyltransferase ST8Sia6, which generated α2,8-linked disialic acids that bind to murine Siglec-E and human Siglec-7 and -9. Increased ST8Sia6 expression was found on many human tumors and associated with decreased survival in several cancers, including colon cancer. Because of this, we engineered MC38 and B16-F10 tumor lines to express ST8Sia6. ST8Sia6-expressing MC38 and B16-F10 tumors exhibited faster growth and led to decreased survival, which required host Siglec-E. ST8Sia6 expression on tumors also altered macrophage polarization toward M2, including upregulation of the immune modulator arginase, which also required Siglec-E. ST8Sia6 also accelerated tumorigenesis in a genetically engineered, spontaneous murine model of colon cancer, decreasing survival from approximately 6 months to 67 days. Thus, ST8Sia6 expression on tumors inhibits antitumor immune responses to accelerate tumor growth.

Abstract The immune system contains a series of checks and balances that maintain tolerance and prevent autoimmunity. Sialic acid-binding Ig-type lectins (Siglecs) are cell surface receptors found on immune cells and inhibit inflammation by recruiting protein tyrosine phosphatases to ITIMs. Islet-resident macrophages express Siglec-E, and Siglec-E expression decreases on islet-resident macrophages as insulitis progresses in the NOD mouse. The sialyltransferase ST8Sia6 generates α-2,8-disialic acids that are ligands for Siglec-E in vivo. We hypothesized that engaging Siglec-E through ST8Sia6-generated ligands may inhibit the development of immune-mediated diabetes. Constitutive overexpression of ST8Sia6 in pancreatic β cells mitigated hyperglycemia in the multiple low-dose streptozotocin model of diabetes, demonstrating that engagement of this immune receptor facilitates tolerance in the setting of inflammation and autoimmune disease.

Abstract NKAP and HDAC3 are critical for T cell maturation. NKAP and HDAC3 physically associate, and a point mutation in NKAP, NKAP(Y352A), abrogates this interaction. To evaluate the significance of NKAP and HDAC3 association in T cell maturation, transgenic mice were engineered for cre-mediated endogenous NKAP gene deletion coupled to induction of NKAP(Y352A) or a wild type (WT) control transgene, NKAP(WT), in double positive thymocytes or regulatory T cells (Tregs). T cell maturation was normal in mice with endogenous NKAP deletion coupled to NKAP(WT) induction. However, severe defects occurred in T cell and Treg maturation and in iNKT cell development when NKAP(Y352A) was induced, recapitulating NKAP deficiency. Conventional T cells expressing NKAP(Y352A) failed to enter the long-term T cell pool, did not produce cytokines, and remained complement susceptible, whereas Tregs expressing NKAP(Y352A) were eliminated as recent thymic emigrants leading to lethal autoimmunity. Overall, these results demonstrate the significance of NKAP–HDAC3 association in T cells.

Abstract Recent thymic emigrants that fail postpositive selection maturation are targeted by complement proteins. T cells likely acquire complement resistance during maturation in the thymus, a complement-privileged organ. To test this, thymocytes and fresh serum were separately obtained and incubated together in vitro to assess complement deposition. Complement binding decreased with development and maturation. Complement binding decreased from the double-positive thymocyte to the single-positive stage, and within single-positive thymocytes, complement binding gradually decreased with increasing intrathymic maturation. Binding of the central complement protein C3 to wild-type immature thymocytes required the lectin but not the classical pathway. Specifically, MBL2 but not MBL1 was required, demonstrating a unique function for MBL2. Previous studies demonstrated that the loss of NKAP, a transcriptional regulator of T cell maturation, caused peripheral T cell lymphopenia and enhanced complement susceptibility. To determine whether complement causes NKAP-deficient T cell disappearance, both the lectin and classical pathways were genetically ablated. This blocked C3 deposition on NKAP-deficient T cells but failed to restore normal cellularity, indicating that complement contributes to clearance but is not the primary cause of peripheral T cell lymphopenia. Rather, the accumulation of lipid peroxides in NKAP-deficient T cells was observed. Lipid peroxidation is a salient feature of ferroptosis, an iron-dependent nonapoptotic cell death. Thus, wild-type thymocytes naturally acquire the ability to protect themselves from complement targeting by MBL2 with maturation. However, NKAP-deficient immature peripheral T cells remain scarce in complement-deficient mice likely due to ferroptosis.

Abstract Intricate life-versus-death decisions are programmed during T cell development, and the regulatory mechanisms that coordinate their activation and repression are still under investigation. In this study, HDAC3-deficient double-positive (DP) thymocytes exhibit a severe decrease in numbers. The thymic cortex is rich in ATP, which is released by macrophages that clear apoptotic DP thymocytes that fail to undergo positive selection. We demonstrate that HDAC3 is required to repress expression of the purinergic receptor P2X7 to prevent DP cell death. HDAC3-deficient DP thymocytes upregulate the P2X7 receptor, increasing sensitivity to ATP-induced cell death. P2rx7/HDAC3-double knockout mice show a partial rescue in DP cell number. HDAC3 directly binds to the P2rx7 enhancer, which is hyperacetylated in the absence of HDAC3. In addition, RORγt binds to the P2rx7 enhancer and promotes P2X7 receptor expression in the absence of HDAC3. Therefore, HDAC3 is a critical regulator of DP thymocyte survival and is required to suppress P2X7 receptor expression.