Regulation of immune responses by inhibitory receptors

The Role of Immune Regulatory Molecules in COVID-19

As the fifth pandemic in the 21st century, coronavirus 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has become the most prominent global concern in the last 2 years. Variable manifestations characterize SARS-CoV-2 infection. Despite the design and production of effective vaccines and their considerable effect on reducing the COVID-19 prevalence and mortality rate, no definitive cure for the disease has yet been found. Mutations may also affect the effectiveness of vaccines. The host immune response to the pathogen has a critical role in the course of the disease. Positive and negative signals often balance the immune system. Immune regulatory molecules, also known as immune checkpoint receptors, balance the immune responses. These molecules mainly have inhibitory functions and prevent hyperactivation of immune cells or trigger adverse signaling pathways. For a decade, the immune checkpoint blockade, as a therapeutic target for cancer immunotherapy, has been utilized. Some of the inhibitory receptors are recognized as exhaustion markers on T cells. The signaling pathway of these markers restricts the function of T cells against viral infection. Dysregulation of T cells was observed in SARS-CoV-2 infection and can modify proliferation, differentiation, cytokine production, and type of response. The pivotal role of immune inhibitory receptors in the function of acquired, cell-mediated, immune defense T cells makes them a fascinating subject to study. This review article summarized recent findings on immune regulatory molecules and their role in SARS-CoV-2 infection, hoping to find a way to design novel treatments.

Natural killer cells: From surface receptors to the cure of high-risk leukemia (Ceppellini Lecture)

Natural killer (NK) cells are innate immune effector cells involved in the first line of defense against viral infections and malignancies. In the last three decades, the identification of HLA class I‐specific inhibitory killer immunoglobulin‐like receptors (KIR) and of the main activating receptors has strongly improved our understanding of the mechanisms regulating NK cell functions. The increased knowledge on how NK cells discriminate healthy cells from damaged cells has made it possible to transfer basic research notions to clinical applications. Of particular relevance is the strong NK‐mediated anti‐leukemia effect in haploidentical hematopoietic stem cell transplantation to cure high‐risk leukemia.

The Roles of Siglec7 and Siglec9 on Natural Killer Cells in Virus Infection and Tumour Progression

The function of natural killer (NK) cells, defending against virus infection and tumour progression, is regulated by multiple activating and inhibiting receptors expressed on NK cells, among which sialic acid-bind immunoglobulin-like lectins (Siglecs) act as a vital inhibitory group. Previous studies have shown that Siglec7 and Siglec9 are expressed on NK cells, which negatively regulate the function of NK cells and modulate the immune response through the interaction of sialic acid-containing ligands. Siglec7 and Siglec9 are very similar in distribution, gene encoding, protein sequences, ligand affinity, and functions in regulating the immune system against virus and cancers, but differences still exist between them. In this review, we aim to discuss the similarities and differences between Siglec7 and Siglec9 and analyze their functions in virus infection and tumour progression in order to develop better anti-viral and anti-tumor immunotherapy in the future.

KIR and KIR ligand polymorphism: a new area for clinical applications?

Killer immunoglobulin-like receptors (KIRs) play an essential role in the regulation of natural killer (NK) activity, allowing NK cells to sense and respond to human leukocyte antigen (HLA) class I downregulation, an important hallmark for viral infections and tumor transformation. KIR and HLA genes are located on different chromosomes and KIR/HLA class I interaction represents an example of genetic epistasis in which the presence of receptor/ligand pairs is necessary for the induction of functional activity, while the presence of one in the absence of the other is not sufficient to influence NK cell function. Due to the high degree of HLA class I and KIR gene variability, KIR/KIR-ligand (KIR-L) interactions are extraordinarily diverse. KIR polymorphism arises from both haplotypic and allelic variations and was shaped by natural selection. KIR variability affects NK cell education influencing the KIR repertoire, KIR expression, the strength of KIR/KIR-L interactions and the capability to deliver signals. Moreover, it may influence NK cell function during infections, autoimmune diseases, pregnancy and allogeneic transplantation. This review summarizes the genetic and functional features of KIR/KIR-L interactions and gives an overview of their potential relevance in clinical studies.

The low affinity Fc receptor for IgG functions as an effective cytolytic receptor for self-specific CD8 T cells

We have recently described a population of self-Ag-specific murine CD8(+) T cells with a memory phenotype that use receptors of both the adaptive and innate immune systems in the detection of transformed and infected cells. In this study we show that upon activation with IL-2 with or without Ag, between 10 and 20% of the activated self-specific CD8(+) T cells express the low affinity FcR for IgG. By contrast, all IL-2-activated NK cells express high levels of this FcR. The FcR comprises the FcgammaRIIIalpha and FcRgamma subunits. However, the FcRgamma subunit also associates with the CD3 complex, and this association probably contributes to the low expression of FcR in activated cells. Although the FcR is expressed at a low level on activated self-specific CD8(+) T cells, it functions very efficiently as a cytolytic receptor in ADCC. FcR-dependent killing occurred in the absence of TCR stimulation, but could be augmented by concurrent stimulation of the TCR. In addition to mediating ADCC, engagement of the FcR on self-specific CD8(+) T cells results in the production of both IFN-gamma and TNF-alpha. This is the first report of an activating FcR on self-specific murine CD8(+)alphabeta TCR(+) T cells and establishes the importance of innate immune system receptors in the function of these self-specific CD8(+) T cells.

Killer immunoglobulin-like receptor expression on single cells: a cautionary note

Natural killer (NK) cells keep the surface expression of major histocompatibility complex (MHC) class I molecules under surveillance using killer immunoglobulin-like receptors (KIR). Virus-infected or aberrant cells are frequently characterized by a reduced surface expression of MHC class I antigens and may therefore be removed by cytolysis. NK cells are heterogeneous with regard to the expression of KIR genes. The resulting subpopulations show distinguishable specificities allowing the recognition of cells lacking varying combinations of MHC class I antigens. The KIR expression pattern in single NK cells has previously been analyzed by Husain and colleagues by cDNA preamplification of CD3- CD56+ single cells and subsequent gene-specific polymerase chain reaction. We show here that the data of this study contain inconsistencies. These inconsistencies are discussed in the context of KIR mRNA abundance and single-cell cDNA amplification efficiency.

Engagement of the inhibitory receptor CD158a interrupts TCR signaling, preventing dynamic membrane reorganization in CTL/tumor cell interaction

Renal cell carcinoma (RCC) infiltrating lymphocytes (TILs) express killer cell immunoglobulinlike receptors (KIRs) that inhibit the antitumor CD8(+) T-cell lysis. In the present study, to better examine the functional consequences of KIR engagement on cytotoxic T lymphocyte (CTL)/tumor interaction, we have investigated the influence of KIR CD158a on early steps of T-cell activation. We show that coengagement of T-cell receptor (TCR) and CD158a by tumor cells inhibited tyrosine phosphorylation of early signaling proteins ZAP-70 and LAT, lipid raft coalescence, and TCR/CD3 accumulation at the CTL/tumor cell interface. In addition, the guanine exchange factor Vav was not phosphorylated, and no actin cytoskeleton rearrangement was observed. Our data indicate a role of KIR CD158a in the dynamic events induced by TCR triggering, preventing CTL membrane reorganization, and subsequent completion of CTL activation program. Accordingly, the expression of CD158 by TILs may favor tumor cell escape to the immune response.

Ig-like transcript 2 (ILT2)/leukocyte Ig-like receptor 1 (LIR1) inhibits TCR signaling and actin cytoskeleton reorganization

Ig-like transcript 2 (ILT2)/leukocyte Ig-like receptor 1 (LIR1) is a receptor, specific for MHC class I molecules, that inhibits lymphoid and myeloid cells. Here, we analyzed the molecular and cellular mechanisms by which ILT2 modulates T cell activation in primary CTLs and transfected T cell lines. We found that cross-linking with the TCR and the activity of Src tyrosine kinase p56(lck) were required for phosphorylation of ILT2 and subsequent recruitment of Src homology protein 1. In contrast, ILT2 triggering resulted in reduced phosphorylation of TCRzeta and linker for activation of T cells, which led to reduced TCRzeta-ZAP70 complex formation, as well as extracellular signal-related kinase 1 and 2 activation. Furthermore, ILT2 inhibited both superantigen and anti-TCR Ab-induced rearrangement of the actin cytoskeleton. The inhibitory effect mediated by ILT2 is probably concentrated at the APC-T cell interface because both TCR and ILT2 were strongly polarized toward the APC upon engagement by their specific ligands. Thus, ILT2 inhibits both signaling and cellular events involved in the activation of T cells.

SHPS-1 is a scaffold for assembling distinct adhesion-regulated multi-protein complexes in macrophages

Inhibitory immunoreceptors downregulate signaling by recruiting Src homology 2 (SH2) domain-containing tyrosine and/or lipid phosphatases to activating receptor complexes [1]. There are indications that some inhibitory receptors might also perform other functions [2] [3]. In adherent macrophages, two inhibitory receptors, SHPS-1 and PIR-B, are the major proteins binding to the tyrosine phosphatase SHP-1. SHPS-1 also associates with two tyrosine-phosphorylated proteins (pp55 and pp130) and a protein tyrosine kinase [4]. Here, we have identified pp55 and pp130 as the adaptor molecules SKAP55hom/R (Src-kinase-associated protein of 55 kDa homologue) and FYB/SLAP-130 (Fyn-binding protein/SLP-76-associated protein of 130 kDa), respectively, and the tyrosine kinase activity as PYK2. Two distinct SHPS-1 complexes were formed, one containing SKAP55hom/R and FYB/SLAP-130, and the other containing PYK2. Recruitment of FYB/SLAP-130 to SHPS-1 required SKAP55hom/R, whereas PYK2 associated with SHPS-1 independently. Formation of both complexes was independent of SHP-1 and tyrosine phosphorylation of SHPS-1. Finally, tyrosine phosphorylation of members of the SHPS-1 complexes was regulated by integrin-mediated adhesion. Thus, SHPS-1 provides a scaffold for the assembly of multi-protein complexes that might both transmit adhesion-regulated signals and help terminate such signals through SHP-1-directed dephosphorylation. Other inhibitory immunoreceptors might have similar scaffold-like functions.

Crystal structure of the human p58 killer cell inhibitory receptor (KIR2DL3) specific for HLA-Cw3-related MHC class I

BACKGROUND

T cells and natural killer (NK) cells perform complementary roles in the cellular immune system. T cells identify infected cells directly through recognition of antigenic peptides that are displayed at the target cell surface by the classical major histocompatibility complex (MHC) class I molecules. NK cells monitor the target cell surface for malfunction of this display system, lysing potentially infected cells that might otherwise evade recognition by the T cells. Human killer cell inhibitory receptors (KIRs) control this process by either inhibiting or activating the cytotoxic activity of NK cells via specific binding to MHC class I molecules on the target cell.

RESULTS

We report the crystal structure of the extracellular region of the human p58 KIR (KIR2DL3), which is specific for the human MHC class I molecule HLA-Cw3 and related alleles. The structure shows the predicted topology of two tandem immunoglobulin-like domains, but comparison with the previously reported structure of the related receptor KIR2DL1 reveals an unexpected change of 23 degrees in the relative orientation of these domains.

CONCLUSIONS

The altered orientation of the immunoglobulin-like domains maintains an unusually acute interdomain elbow angle, which therefore appears to be a distinctive feature of the KIRs. The putative MHC class I binding site is located on the outer surface of the elbow, spanning both domains. The unexpected observation that this binding site can be modulated by differences in the relative domain orientations has implications for the general mechanism of KIR-MHC class I complex formation.