MR1, an immunological periscope of cellular metabolism

Functional and biological implications of clonotypic diversity among human donor-unrestricted T cells

T cells express a T-cell receptor (TCR) heterodimer that is the product of germline rearrangement and junctional editing resulting in immense clonotypic diversity. The generation of diverse TCR repertoires enables the recognition of pathogen-derived peptide antigens presented by polymorphic major histocompatibility complex (MHC) molecules. However, T cells also recognize nonpeptide antigens through nearly monomorphic antigen-presenting systems, such as cluster of differentiation 1 (CD1), MHC-related protein 1 (MR1) and butyrophilins (BTNs). This potential for shared immune responses across genetically diverse populations led to their designation as donor-unrestricted T cells (DURTs). As might be expected, some CD1-, MR1- and BTN-restricted T cells express a TCR that is conserved across unrelated individuals. However, several recent studies have reported unexpected diversity among DURT TCRs, and increasing evidence suggests that this diversity has functional consequences. Recent reports also challenge the dogma that immune cells are either innate or adaptive and suggest that DURT TCRs may act in both capacities. Here, we review this evidence and propose an expanded view of the role for clonotypic diversity among DURTs in humans, including new perspectives on how DURT TCRs may integrate their adaptive and innate immune functions.

The MR1/MAIT cell axis in CNS diseases

Mucosal-associated invariant T (MAIT) cells are a subpopulation of innate-like T cells that can be found throughout the body, predominantly in mucosal sites, the lungs and in the peripheral blood. MAIT cells recognize microbial-derived vitamin B (e.g., riboflavin) metabolite antigens that are presented by the major histocompatibility complex class I-like protein, MR1, found on a variety of cell types in the periphery and the CNS. Since their original discovery, MAIT cells have been studied predominantly in their roles in diseases in the periphery; however, it was not until the early 2000s that these cells were first examined for their contributions to disorders of the CNS, with the bulk of the work being done within the past few years. Currently, the MR1/MAIT cell axis has been investigated in only a few neurological diseases including, multiple sclerosis and experimental autoimmune encephalomyelitis, brain cancer/tumors, ischemia, cerebral palsy, general aging and, most recently, Alzheimer's disease. Each of these diseases demonstrates a role for this under-studied innate immune axis in its neuropathology. Together, they highlight the importance of studying the MR1/MAIT cell axis in CNS disorders. Here, we review the contributions of the MR1/MAIT cell axis in the progression or remission of these neurological diseases. This work has shed some light in terms of potentially exploiting the MR1/MAIT cell axis in novel therapeutic applications.

Natural killer T cells and other innate-like T lymphocytes as emerging platforms for allogeneic cancer cell therapy

T cells expressing chimeric antigen receptors (CARs) have achieved major clinical success in patients with hematologic malignancies. However, these treatments remain largely ineffective for solid cancers and require significant time and resources to be manufactured in an autologous setting. Developing alternative immune effector cells as cancer immunotherapy agents that can be employed in allogeneic settings is crucial for the advancement of cell therapy. Unlike T cells, Vα24-invariant natural killer T cells (NKTs) are not alloreactive and can therefore be generated from allogeneic donors for rapid infusion into numerous patients without the risk of graft-versus-host disease. Additionally, NKT cells demonstrate inherent advantages over T-cell products, including the ability to traffic to tumor tissues, target tumor-associated macrophages, transactivate NK cells, and cross-prime tumor-specific CD8 T cells. Both unmodified NKTs, which specifically recognize CD1d-bound glycolipid antigens expressed by certain types of tumors, and CAR-redirected NKTs are being developed as the next generation of allogeneic cell therapy products. In this review, we describe studies on the biology of NKTs and other types of innate-like T cells and summarize the clinical experiences of unmodified and CAR-redirected NKTs, including recent interim reports on allogeneic NKTs.

Translatome analyses by bio-orthogonal non-canonical amino acid labeling reveal that MR1-activated MAIT cells induce an M1 phenotype and antiviral programming in antigen-presenting monocytes

MAIT cells are multifunctional innate-like effector cells recognizing bacterial-derived vitamin B metabolites presented by the non-polymorphic MHC class I related protein 1 (MR1). However, our understanding of MR1-mediated responses of MAIT cells upon their interaction with other immune cells is still incomplete. Here, we performed the first translatome study of primary human MAIT cells interacting with THP-1 monocytes in a bicellular system. We analyzed the interaction between MAIT and THP-1 cells in the presence of the activating 5-OP-RU or the inhibitory Ac-6-FP MR1-ligand. Using bio-orthogonal non-canonical amino acid tagging (BONCAT) we were able to enrich selectively those proteins that were newly translated during MR1-dependent cellular interaction. Subsequently, newly translated proteins were measured cell-type-specifically by ultrasensitive proteomics to decipher the coinciding immune responses in both cell types. This strategy identified over 2,000 MAIT and 3,000 THP-1 active protein translations following MR1 ligand stimulations. Translation in both cell types was found to be increased by 5-OP-RU, which correlated with their conjugation frequency and CD3 polarization at MAIT cell immunological synapses in the presence of 5-OP-RU. In contrast, Ac-6-FP only regulated a few protein translations, including GSK3B, indicating an anergic phenotype. In addition to known effector responses, 5-OP-RU-induced protein translations uncovered type I and type II Interferon-driven protein expression profiles in both MAIT and THP-1 cells. Interestingly, the translatome of THP-1 cells suggested that activated MAIT cells can impact M1/M2 polarization in these cells. Indeed, gene and surface expression of CXCL10, IL-1β, CD80, and CD206 confirmed an M1-like phenotype of macrophages being induced in the presence of 5-OP-RU-activated MAIT cells. Furthermore, we validated that the Interferon-driven translatome was accompanied by the induction of an antiviral phenotype in THP-1 cells, which were found able to suppress viral replication following conjugation with MR1-activated MAIT cells. In conclusion, BONCAT translatomics extended our knowledge of MAIT cell immune responses at the protein level and discovered that MR1-activated MAIT cells are sufficient to induce M1 polarization and an anti-viral program of macrophages.