Asymptomatic SARS-CoV-2 infection: is it all about being refractile to innate immune sensing of viral spare-parts?-Clues from exotic animal reservoirs.

A vast proportion of coronavirus disease 2019 (COVID-19) individuals remain asymptomatic and can shed severe acute respiratory syndrome (SARS-CoV) type 2 virus to transmit the infection, which also explains the exponential increase in the number of COVID-19 cases globally. Furthermore, the rate of recovery from clinical COVID-19 in certain pockets of the globe is surprisingly high. Based on published reports and available literature, here, we speculated a few immunovirological mechanisms as to why a vast majority of individuals remain asymptomatic similar to exotic animal (bats and pangolins) reservoirs that remain refractile to disease development despite carrying a huge load of diverse insidious viral species, and whether such evolutionary advantage would unveil therapeutic strategies against COVID-19 infection in humans. Understanding the unique mechanisms that exotic animal species employ to achieve viral control, as well as inflammatory regulation, appears to hold key clues to the development of therapeutic versatility against COVID-19.

Killer-like receptors and GPR56 progressive expression defines cytokine production of human CD4+ memory T cells.

All memory T cells mount an accelerated response on antigen reencounter, but significant functional heterogeneity is present within the respective memory T-cell subsets as defined by CCR7 and CD45RA expression, thereby warranting further stratification. Here we show that several surface markers, including KLRB1, KLRG1, GPR56, and KLRF1, help define low, high, or exhausted cytokine producers within human peripheral and intrahepatic CD4+ memory T-cell populations. Highest simultaneous production of TNF and IFN-γ is observed in KLRB1+KLRG1+GPR56+ CD4 T cells. By contrast, KLRF1 expression is associated with T-cell exhaustion and reduced TNF/IFN-γ production. Lastly, TCRß repertoire analysis and in vitro differentiation support a regulated, progressive expression for these markers during CD4+ memory T-cell differentiation. Our results thus help refine the classification of human memory T cells to provide insights on inflammatory disease progression and immunotherapy development.

The Activating C-type Lectin-like Receptor NKp65 Signals through a Hemi-immunoreceptor Tyrosine-based Activation Motif (hemITAM) and Spleen Tyrosine Kinase (Syk).

NKp65 is an activating human C-type lectin-like receptor (CTLR) triggering cellular cytotoxicity and cytokine secretion upon high-affinity interaction with the cognate CTLR keratinocyte-associated C-type lectin (KACL) selectively expressed by human keratinocytes. Previously, we demonstrated that NKp65-mediated cellular cytotoxicity depends on tyrosine 7, located in a cytoplasmic sequence motif of NKp65 resembling a hemi-immunoreceptor tyrosine-based activation motif (hemITAM). HemITAMs have been reported for a few activating myeloid-specific CTLRs, including Dectin-1 and CLEC-2, and consist of a single tyrosine signaling unit preceded by a triacidic motif. Upon receptor engagement, the hemITAM undergoes phosphotyrosinylation and specifically recruits spleen tyrosine kinase (Syk), initiating cellular activation. In this study, we addressed the functionality of the putative hemITAM of NKp65. We show that NKp65 forms homodimers and is phosphorylated at the hemITAM-embedded tyrosine 7 upon engagement by antibodies or KACL homodimers. HemITAM phosphotyrosinylation initiates a signaling pathway involving and depending on Syk, leading to cellular activation and natural killer (NK) cell degranulation. However, although NKp65 utilizes Syk for NK cell activation, a physical association of Syk with the NKp65 hemITAM could not be detected, unlike shown previously for the hemITAM of myeloid CTLR. Failure of NKp65 to recruit Syk is not due to an alteration of the triacidic motif, which rather affects the efficiency of hemITAM phosphotyrosinylation. In summary, NKp65 utilizes a hemITAM-like motif for cellular activation that requires Syk, although Syk appears not to be recruited to NKp65.

Innate lymphoid cells and the MHC.

Innate lymphoid cells (ILCs) are a new class of immune cells that include natural killer (NK) cells and appear to be the innate counterparts to CD4(+) helper T cells and CD8(+) cytotoxic T cells based on developmental and functional similarities. Like T cells, both NK cells and other ILCs also show connections to the major histocompatibility complex (MHC). In human and mouse, NK cells recognize and respond to classical and nonclassical MHC I molecules as well as structural homologues, whereas mouse ILCs have recently been shown to express MHC II. We describe the history of MHC I recognition by NK cells and discuss emerging roles for MHC II expression by ILC subsets, making comparisons between both mouse and human when possible.

NK cells: tuned by peptide?

Natural killer cells express multiple receptors for major histocompatibility complex (MHC) class I, including the killer cell immunoglobulin-like receptors (KIRs) and the C-type lectin-like CD94:NKG2 receptors. The KIR locus is extremely polymorphic, paralleling the diversity of its classical MHC class I ligands. Similarly, the conservation of the NKG2 family of receptors parallels the conservation of MHC-E, the ligand for CD94:NKG2A/C/E. Binding of both CD94:NKG2 heterodimers and KIR to their respective MHC class I ligand is peptide dependent, and despite the evolution of these receptors, they have retained the property of peptide selectivity. Such peptide selectivity affects these two systems in different ways. HLA-E binding non-inhibitory peptides augment inhibition at CD94:NKG2A, while HLA-C binding non-inhibitory peptides antagonize inhibition at KIR2DL2/3, implying that KIRs are specialized to respond positively to changes in peptide repertoire. Thus, while specific KIRs, such as KIR2DL3, are associated with beneficial outcomes from viral infections, viral peptides augment inhibition at CD94:NKGA. Conversely, NKG2A-positive NK cells sense MHC class I downregulation more efficiently than KIRs. Thus, these two receptor:ligand systems appear to have complementary functions in recognizing changes in MHC class I.

Cattle NK Cell Heterogeneity and the Influence of MHC Class I.

Primate and rodent NK cells form highly heterogeneous lymphocyte populations owing to the differential expression of germline-encoded receptors. Many of these receptors are polymorphic and recognize equally polymorphic determinants of MHC class I. This diversity can lead to individuals carrying NK cells with different specificities. Cattle have an unusually diverse repertoire of NK cell receptor genes predicted to encode receptors that recognize MHC class I. To begin to examine whether this genetic diversity leads to a diverse NK cell population, we isolated peripheral NK cells from cattle with different MHC homozygous genotypes. Cytokine stimulation differentially influenced the transcription of five receptors at the cell population level. Using dilution cultures, we found that a further seven receptors were differentially transcribed, including five predicted to recognize MHC class I. Moreover, there was a statistically significant reduction in killer cell lectin-like receptor mRNA expression between cultures with different CD2 phenotypes and from animals with different MHC class I haplotypes. This finding confirms that cattle NK cells are a heterogeneous population and reveals that the receptors creating this diversity are influenced by the MHC. The importance of this heterogeneity will become clear as we learn more about the role of NK cells in cattle disease resistance and vaccination.

Key residues at the membrane-distal surface of KACL, but not glycosylation, determine the functional interaction of the keratinocyte-specific C-type lectin-like receptor KACL with its high-affinity receptor NKp65.

Keratinocyte-associated C-type lectin (KACL) is a peculiar C-type lectin-like receptor (CTLR) due to its selective expression by human keratinocytes and cognate interaction with the genetically coupled CTLR NKp65. KACL and NKp65 are members of the CLEC2 and NKRP1 subfamilies of natural killer gene complex (NKC)-encoded CTLR, respectively. Most NKRP1 molecules are expressed on NK cells and T cells and act as receptors of CLEC2 glycoproteins with their genes being intermingled in a certain sub-region of the mammalian NKC. The reasons for the tight genetic linkage of these dedicated receptor/ligand pairs are unknown, as is the physiological expression of NKp65. Recently, we reported that the CTLR NKp65 and KACL interact with high affinity, resulting in activation of NKp65-expressing NK-92MI cells. Here, we address the molecular basis of this high-affinity interaction by analysing KACL mutants with KACL-specific monoclonal antibodies (mAb), soluble NKp65 (sNKp65) and NK-92MI-NKp65 cells. We find that none of the three N-linked carbohydrates of KACL glycoproteins significantly contributes to KACL surface expression and NKp65 interaction. However, KACL mutants with non-conservative amino acid substitutions of arginine 158 or isoleucine 161 abrogated binding of both KACL-specific mAb OMA1 and sNKp65, well in line with the blockade of NKp65-KACL interaction by OMA1. Accordingly, functional recognition of these KACL mutants by NK-92M-NKp65 cells was completely abolished. Arginine 158 and isoleucine 161 located at the membrane-distal surface of KACL were defined as residues, decisively determining functional KACL-NKp65 interaction that is independent of KACL glycosylation.

Enhanced cytotoxic function of natural killer and CD3+CD56+ cells in cord blood after culture.

Rate of immune reconstitution directly correlates with the number of hematopoietic stem cells infused and is particularly delayed in patients undergoing cord blood (CB) transplantation (CBT). Methods to increase the number of CB natural killer (NK) cells have the potential to improve immune reconstitution after CBT. NK cells are the first lymphocyte population to recover after hematopoietic stem cells transplantation and are central to preventing early relapse and infection. CB NK cells are low in number and are known to be incomplete in maturation and require activation for effective function. Here, we report a clinically relevant ex vivo expansion method that increases the number of activated CB NK cells. We report a multilog increase in NK cell number when CB mononuclear cells are cocultured with IL-2 and IL-15. Furthermore, NK cells expressing activating receptors and adhesion molecules responsible for cytotoxicity increased throughout culture, whereas inhibitory receptor expression remained low. Additionally, cytotoxic function against various malignancies was significantly enhanced in cultured NK cells but not CD3(+)CD56(+) cells. These data suggest that ex vivo expansion and activation of CB NK cells is a clinically feasible and relevant approach to prevent early infection and relapse after CBT.

[Mast cell inhibitory receptors]. / Receptory hamujace komórek tucznych.

 Mast cells play an important role in diverse physiological mechanisms as well as taking part in antimicrobial defense. What is more, these cells are important regulators of a number of pathophysiological processes, involving allergic reactions. Therefore, it seems to be very important to know and understand the factors and receptors influencing mast cell activity. Nowadays it is well established that activating signals are counterbalanced by negative or inhibition signals transmitted by inhibitory receptors containing immunoreceptor tyrosine-based inhibitory motifs (ITIMs). Inhibitory receptor engagement leads to ITIM tyrosine phosphorylation, the recruitment and activation of protein tyrosine phosphatases such as SHP-1, SHP-2 and/or SHIP, and the dephosphorylation of activating receptor associated proteins. There is growing evidence that a number of inhibitory receptors have been identified on mast cells. The scope of this paper is to present the current knowledge on mast cell-associated inhibitory receptors, such as FcγRIIB, paired immunoglobulin-like receptor B (PIR-B), CD300, CD172a, gp49B1, CD200R, sialic acid-binding immunoglobulin-like lectin (Siglec) molecules, CD305, allergin-1, mast cell function-associated antigen (MAFA), and CD72. The role of these inhibitory receptors in regulation mast cell activity is also discussed.

[The role of HLA-E polymorphism in immunological response]. / Znaczenie polimorfizmu HLA-E w odpowiedzi immunologicznej.

The HLA-E protein is one of the most extensively studied MHC class Ib antigens and the least polymorphic one compared to other MHC class I molecules. In the human population there have been reported just ten alleles encoding three different peptides. Only two of these alleles, namely HLA-E*0101 and HLA-E*0103, are widely distributed (around 50% each). The proteins encoded by these alleles differ from each other in one amino acid at position 107. In HLA-E*0101 it is arginine and in HLA-E*0103 it is glycine. The difference between these proteins manifests itself in surface expression levels, affinities to leader peptides and thermal stabilities of their complexes. The HLA-E molecule is a ligand for CD94/NKG2 receptors on NK cells and TCR receptors on NK-CTL (NK-cytotoxic T lymphocyte) cells, so it plays a double role in both innate and adaptive immunity. This paper reviews the knowledge on the role of the HLA-E molecule in the immunological response. Aspects related to polymorphism of the HLA-E gene and the course of several diseases including type I diabetes, ankylosing spondylitis, HCV and HIV infections, nasopharyngeal cancer and recurrent spontaneous abortions, as well as the outcome of hematopoietic stem cell transplantation, are presented and discussed in more detail.

Natural killer cell tolerance licensing and other mechanisms.

Armed with potent cytotoxic and immunostimulatory effector functions, natural killer (NK) cells have the potential to cause significant damage to normal self cells unless controlled by self-tolerance mechanisms. NK cells identify and attack target cells based on integration of signals from activation and inhibitory receptors, whose ligands exhibit complex expression and/or binding patterns. Preservation of NK cell self-tolerance must therefore go beyond mere engagement of inhibitory receptors during effector functions. Herein, we review recent work that has uncovered a number of mechanisms to ensure self-tolerance of NK cells. For example, licensing of NK cells allows only NK cells that can engage self-MHC to become functionally competent, or licensed. The molecular mechanism of this phenomenon appears to require signaling by receptors that were originally identified in effector inhibition. However, the nature of the signaling event has not yet been defined, but new interpretations of several published experiments provide valuable clues. In addition, several other cell-intrinsic and -extrinsic mechanisms of NK cell tolerance are discussed, including activation receptor cooperation and synergy, cytokine stimulation, and the opposing roles of accessory and regulatory cells. Finally, NK cell tolerance is discussed as it relates to the clinic, such as KIR-HLA disease associations, tumor immunotherapy, and fetal tolerance.