Microparticle Delivery of a STING Agonist Enables Indirect Activation of NK Cells by Antigen-Presenting Cells.

Natural killer (NK) cells are an important member of the innate immune system and can participate in direct tumor cell killing in response to immunotherapies. One class of immunotherapy is stimulator of interferon gene (STING) agonists, which result in a robust type I interferon (IFN-I) response. Most mechanistic studies involving STING have focused on macrophages and T cells. Nevertheless, NK cells are also activated by IFN-I, but the effect of STING activation on NK cells remains to be adequately investigated. We show that both direct treatment with soluble STING agonist cyclic di-guanosine monophosphate-adenosine monophosphate (cGAMP) and indirect treatment with cGAMP encapsulated in microparticles (MPs) result in NK cell activation in vitro, although the former requires 100× more cGAMP than the latter. Additionally, direct activation with cGAMP leads to NK cell death. Indirect activation with cGAMP MPs does not result in NK cell death but rather cell activation and cell killing in vitro. In vivo, treatment with soluble cGAMP and cGAMP MPs both cause short-term activation, whereas only cGAMP MP treatment produces long-term changes in NK cell activation markers. Thus, this work indicates that treatment with an encapsulated STING agonist activates NK cells more efficiently than that with soluble cGAMP. In both the in vitro and in vivo systems, the MP delivery system results in more robust effects at a greatly reduced dosage. These results have potential applications in aiding the improvement of cancer immunotherapies.

MERTK on mononuclear phagocytes regulates T cell antigen recognition at autoimmune and tumor sites.

Understanding mechanisms of immune regulation is key to developing immunotherapies for autoimmunity and cancer. We examined the role of mononuclear phagocytes during peripheral T cell regulation in type 1 diabetes and melanoma. MERTK expression and activity in mononuclear phagocytes in the pancreatic islets promoted islet T cell regulation, resulting in reduced sensitivity of T cell scanning for cognate antigen in prediabetic islets. MERTK-dependent regulation led to reduced T cell activation and effector function at the disease site in islets and prevented rapid progression of type 1 diabetes. In human islets, MERTK-expressing cells were increased in remaining insulin-containing islets of type 1 diabetic patients, suggesting that MERTK protects islets from autoimmune destruction. MERTK also regulated T cell arrest in melanoma tumors. These data indicate that MERTK signaling in mononuclear phagocytes drives T cell regulation at inflammatory disease sites in peripheral tissues through a mechanism that reduces the sensitivity of scanning for antigen leading to reduced responsiveness to antigen.

Landscape and selection of vaccine epitopes in SARS-CoV-2.

BACKGROUND: Early in the pandemic, we designed a SARS-CoV-2 peptide vaccine containing epitope regions optimized for concurrent B cell, CD4+ T cell, and CD8+ T cell stimulation. The rationale for this design was to drive both humoral and cellular immunity with high specificity while avoiding undesired effects such as antibody-dependent enhancement (ADE). METHODS: We explored the set of computationally predicted SARS-CoV-2 HLA-I and HLA-II ligands, examining protein source, concurrent human/murine coverage, and population coverage. Beyond MHC affinity, T cell vaccine candidates were further refined by predicted immunogenicity, sequence conservation, source protein abundance, and coverage of high frequency HLA alleles. B cell epitope regions were chosen from linear epitope mapping studies of convalescent patient serum, followed by filtering for surface accessibility, sequence conservation, spatial localization near functional domains of the spike glycoprotein, and avoidance of glycosylation sites. RESULTS: From 58 initial candidates, three B cell epitope regions were identified. From 3730 (MHC-I) and 5045 (MHC-II) candidate ligands, 292 CD8+ and 284 CD4+ T cell epitopes were identified. By combining these B cell and T cell analyses, as well as a manufacturability heuristic, we proposed a set of 22 SARS-CoV-2 vaccine peptides for use in subsequent murine studies. We curated a dataset of ~ 1000 observed T cell epitopes from convalescent COVID-19 patients across eight studies, showing 8/15 recurrent epitope regions to overlap with at least one of our candidate peptides. Of the 22 candidate vaccine peptides, 16 (n = 10 T cell epitope optimized; n = 6 B cell epitope optimized) were manually selected to decrease their degree of sequence overlap and then synthesized. The immunogenicity of the synthesized vaccine peptides was validated using ELISpot and ELISA following murine vaccination. Strong T cell responses were observed in 7/10 T cell epitope optimized peptides following vaccination. Humoral responses were deficient, likely due to the unrestricted conformational space inhabited by linear vaccine peptides. CONCLUSIONS: Overall, we find our selection process and vaccine formulation to be appropriate for identifying T cell epitopes and eliciting T cell responses against those epitopes. Further studies are needed to optimize prediction and induction of B cell responses, as well as study the protective capacity of predicted T and B cell epitopes.

Immune privilege of the CNS is not the consequence of limited antigen sampling.

Central nervous system (CNS) immune privilege is complex, and it is still not understood how CNS antigens are sampled by the peripheral immune system under steady state conditions. To compare antigen sampling from immune-privileged or nonprivileged tissues, we created transgenic mice with oligodendrocyte or gut epithelial cell expression of an EGFP-tagged fusion protein containing ovalbumin (OVA) antigenic peptides and tested peripheral anti-OVA peptide-specific sentinel OT-I and OT-II T cell activation. We report that oligodendrocyte or gut antigens are sampled similarly, as determined by comparable levels of OT-I T cell activation. However, activated T cells do not access the CNS under steady state conditions. These data show that afferent immunity is normally intact as there is no barrier at the antigen sampling level, but that efferent immunity is restricted. To understand how this one-sided surveillance contributes to CNS immune privilege will help us define mechanisms of CNS autoimmune disease initiation.