The Invariant NKT Cell Response Has Differential Signaling Requirements during Antigen-Dependent and Antigen-Independent Activation.

Invariant NKT (iNKT) cells are an innate-like population characterized by their recognition of glycolipid Ags and rapid cytokine production upon activation. Unlike conventional T cells, which require TCR ligation, iNKT cells can also be stimulated independently of their TCR. This feature allows iNKT cells to respond even in the absence of glycolipid Ags, for example, during viral infections. Although the TCR-dependent and -independent activation of iNKT cells have been relatively well established, the exact contributions of IL-12, IL-18, and TLRs remain unclear for these two activation pathways. To definitively investigate how these components affect the direct and indirect stimulation of iNKT cells, we used mice deficient for either MyD88 or the IL-12Rß2 in the T cell lineage. Using these tools, we demonstrate that IL-12, IL-18, and TLRs are completely dispensable for the TCR activation pathway when a strong agonist is used. In contrast, during murine CMV infection, when the TCR is not engaged, IL-12 signaling is essential, and TLR signaling is expendable. Importantly, to our knowledge, we discovered an intrinsic requirement for IL-18 signaling by splenic iNKT cells but not liver iNKT cells, suggesting that there might be diversity, even within the NKT1 population.

NFIL3 Expression Distinguishes Tissue-Resident NK Cells and Conventional NK-like Cells in the Mouse Submandibular Glands.

The submandibular salivary gland (SMG), a major site of persistent infection for many viruses, contains a large NK cell population. Using NFIL3-deficient mice, PLZF reporter/fate mapping mice, and mixed bone marrow chimeras, we identified two distinct populations of NK cells in the SMG. Although phenotypically unique, the main population relies on NFIL3, but not PLZF, for development and, therefore, is developmentally similar to the conventional NK cell subset. In contrast, we found that approximately one quarter of the SMG NK cells develop independently of NFIL3. Interestingly, NFIL3-independent SMG tissue-resident NK (trNK) cells are developmentally distinct from liver trNK cells. We also demonstrated that the SMG NK cell hyporesponsive phenotype during murine CMV infection is tissue specific and not cell intrinsic. In contrast, NFIL3-independent SMG trNK cells are intrinsically hyporesponsive. Altogether, our data show that the SMG tissue environment shapes a unique repertoire of NK-like cells with distinct phenotypes.

Role of SHIP1 in Invariant NKT Cell Development and Functions.

SHIP1 is a 5'-inositol phosphatase known to negatively regulate the signaling product of the PI3K pathway, phosphatidylinositol (3-5)-trisphosphate. SHIP1 is recruited to a large number of inhibitory receptors expressed on invariant NK (iNKT) cells. We hypothesized that SHIP1 deletion would have major effects on iNKT cell development by altering the thresholds for positive and negative selection. Germline SHIP1 deletion has been shown to affect T cells as well as other immune cell populations. However, the role of SHIP1 on T cell function has been controversial, and its participation on iNKT cell development and function has not been examined. We evaluated the consequences of SHIP1 deletion on iNKT cells using germline-deficient mice, chimeric mice, and conditionally deficient mice. We found that T cell and iNKT cell development are impaired in germline-deficient animals. However, this phenotype can be rescued by extrinsic expression of SHIP1. In contrast, SHIP1 is required cell autonomously for optimal iNKT cell cytokine secretion. This suggests that SHIP1 calibrates the threshold of iNKT cell reactivity. These data further our understanding of how iNKT cell activation is regulated and provide insights into the biology of this unique cell lineage.

The role of MHC class Ib-restricted T cells during infection.

Even though major histocompatibility complex (MHC) class Ia and many Ib molecules have similarities in structure, MHC class Ib molecules tend to have more specialized functions, which include the presentation of non-peptidic antigens to non-classical T cells. Likewise, non-classical T cells also have unique characteristics, including an innate-like phenotype in naïve animals and rapid effector functions. In this review, we discuss the role of MAIT and NKT cells during infection but also the contribution of less studied MHC class Ib-restricted T cells such as Qa-1-, Qa-2-, and M3-restricted T cells. We focus on describing the types of antigens presented to non-classical T cells, their response and cytokine profile following infection, as well as the overall impact of these T cells to the immune system.

Qa-1-Restricted CD8+ T Cells Can Compensate for the Absence of Conventional T Cells during Viral Infection.

The role of non-classical T cells during viral infection remains poorly understood. Using the well-established murine model of CMV infection (MCMV) and mice deficient in MHC class Ia molecules, we found that non-classical CD8+ T cells robustly expand after MCMV challenge, become highly activated effectors, and are capable of forming durable memory. Interestingly, although these cells are restricted by MHC class Ib molecules, they respond similarly to conventional T cells. Remarkably, when acting as the sole component of the adaptive immune response, non-classical CD8+ T cells are sufficient to protect against MCMV-induced lethality. We also demonstrate that the MHC class Ib molecule Qa-1 (encoded by H2-T23) restricts a large, and critical, portion of this population. These findings reveal a potential adaptation of the host immune response to compensate for viral evasion of classical T cell immunity.