In jawed vertebrates, the heterogeneous nonclassical MHC class Ib (class Ib) gene family encodes molecules structurally similar to classical MHC class Ia (class Ia) but with more limited tissue distribution and lower polymorphism. In mammals, class Ib gene products are involved in stress responses, malignancy and differentiation of intrathymic CD8 T cells. The frog Xenopus laevis possesses at least 20 class Ib genes (XNCs), and 9 subfamilies have been defined so far. We have characterized two novel subfamilies, XNC10 and XNC11. XNC10 is phylogenetically and structurally distinct from both class Ia and other XNC genes. Besides thymic lymphoid tumors, XNC10 is preferentially expressed by circulating T cells and thymocytes of the CD8 lineage both in adult and in larvae from the onset of thymus organogenesis. XNC11 is expressed only by thymocytes and upregulated by several thymic lymphoid tumors. These data provide the first evidence of the expression of any class Ib genes in Xenopus larvae, and suggests evolutionary relationships between certain class Ib genes, malignancy and CD8 T cell ontogeny.

In the absence of virus-targeting small-molecule drugs approved for the treatment and prevention of COVID-19, broadening the repertoire of potent SARS-CoV-2-neutralizing antibodies represents an important area of research in response to the ongoing pandemic. Systematic analysis of such antibodies and their combinations can be particularly instrumental for identification of candidates that may prove resistant to the emerging viral escape variants. Here, we isolated a panel of 23 RBD-specific human monoclonal antibodies from the B cells of convalescent patients. A surprisingly large proportion of such antibodies displayed potent virus-neutralizing activity both in vitro and in vivo. Four of the isolated nAbs can be categorized as ultrapotent with an apparent IC100 below 16 ng/mL. We show that individual nAbs as well as dual combinations thereof retain activity against currently circulating SARS-CoV-2 variants of concern (such as B.1.1.7, B.1.351, B.1.617, and C.37), as well as against other viral variants. When used as a prophylactics or therapeutics, these nAbs could potently suppress viral replication and prevent lung pathology in SARS-CoV-2-infected hamsters. Our data contribute to the rational development of oligoclonal therapeutic nAb cocktails mitigating the risk of SARS-CoV-2 escape.

Nonclassical MHC class Ib (class Ib) genes are found in all jawed vertebrates, and their products are hypothesized to be indicators of intracellular stress and malignancy. They may be involved in immune recognition of classical MHC class Ia (class Ia)‐low or ‐negative tumor cells through their interaction with T cell receptors and/or non‐T cell inhibitory or triggering receptors expressed by NK cells and T cells. In the frog Xenopus, the molecular chaperone gp96 mediates a potent immune response involving antigen‐specific classical class Ia‐unrestricted CD8+ CTL (CCU‐CTL) against a transplantable thymic tumor (15/0) that does not express class Ia molecules. We hypothesized that Xenopus nonclassical class Ib gene products (XNC) are involved in gp96‐mediated CCU‐CTL anti‐tumor responses. To investigate the involvement of class Ib gene products in Xenopus anti‐tumor responses, we generated, for the first time in ectothermic vertebrates, stable tumor transfectants expressing short hairpin RNA (shRNA) to silence either XNC directly or β2m to prevent class Ib surface expression. Both types of 15/0 transfectants are more resistant to CCU‐CTL killing, more tumorigenic and more susceptible to NK‐like cell killing. This study provides in vitro and in vivo evidence of the evolutionary conservation of class Ib involvement in anti‐tumor CD8+ T cell responses.

BackgroundRecent studies have revealed an unexpected diversity of domain architecture among FcR-like receptors that presumably fulfill regulatory functions in the immune system. Different species of mammals, as well as chicken and catfish have been found to possess strikingly different sets of these receptors. To better understand the evolutionary history of paired receptors, we extended the study of FcR-like genes in amphibian representatives Xenopus tropicalis and Xenopus laevis.ResultsThe diploid genome of X. tropicalis contains at least 75 genes encoding paired FcR-related receptors designated XFLs. The allotetraploid X. laevis displays many similar genes primarily expressed in lymphoid tissues. Up to 35 domain architectures generated by combinatorial joining of six Ig-domain subtypes and two subtypes of the transmembrane regions were found in XFLs. None of these variants are shared by FcR-related proteins from other studied species. Putative activating XFLs associate with the FcRγ subunit, and their transmembrane domains are highly similar to those of activating mammalian KIR-related receptors. This argues in favor of a common origin for the FcR and the KIR families. Phylogenetic analysis shows that the entire repertoires of the Xenopus and mammalian FcR-related proteins have emerged after the amphibian-amniotes split.ConclusionFcR- and KIR-related receptors evolved through continual species-specific diversification, most likely by extensive domain shuffling and birth-and-death processes. This mode of evolution raises the possibility that the ancestral function of these paired receptors was a direct interaction with pathogens and that many physiological functions found in the mammalian receptors were secondary acquisitions or specializations.

Receptors subdivided into inhibitory and activating forms play important roles in the regulation of leukocyte development and effector functions. Two prototypic examples of paired receptors are Fc-receptors (FcR) and Killer cell Immunoglobulin-like receptors (KIR). FcRs are cell surface proteins that bind to the constant regions of IgG and IgE. Classical KIRs recognize MHC class I molecules and regulate natural killer (NK) cell cytotoxic functions. The evolution of these proteins and the time of their origin remain enigmatic. So far, molecules unequivocally related to mammalian FcRs and KIRs have been identified in chicken and an amphibian Xenopus. The lineage-specific evolution of the FcR and KIR families apparently led to the generation of unique sets of receptors in all species studied. Members of both families show extraordinary diversity of domain architectures. This structural diversity makes elusive the functional relationships between the highly specialized mammalian FcR and KIR genes and their homologs in nonmammalian species.