S Rojo

Killer cell inhibitory receptors: diversity, specificity, and function

Summary: NK cells selectively kill target cells that fail to express self‐MHC class I molecules. This selective killing results from a balance between inhibitory NK receptors specific for MHC class I molecules and activating receptors that are still largely unknown. Isolation of molecular clones for the human killer cell inhibitory receptors (KIR) revealed that KIR consist of a family of molecules with Ig ectodomains and cytoplasmic tails o f varying length. Soluble complexes of KIR and HLA‐C molecules established that KIR recognizes and binds to its ligand as an autonomous receptor. A functional expression system in human NK clones demonstrated that a single KIR can provide both recognition of MHC class 1 and delivery of a dominant negative signal to the NK cell. Functional evidence has been obtained for a role of Uie tyrosine phosphatase SHP‐1 in KIR‐mediated inhibition. The presence of a conserved molif used to recruit and activate SHP‐1 in the cytoplasmic tail of KIR and of the mouse Ly‐49 inhibitory receptor (otherwise structurally unrelated to KIR) represents an interesting case of evolutionary convergence. Furthermore, the motif led to the identification of other receptors with inhibitory potential, including a type I Ig‐like receptor shared by mouse mast cells and NK cells.

Natural Killer Cells and Mast Cells from gp49B Null Mutant Mice Are Functional

ABSTRACT Immune responses are controlled by a combination of positive and negative cellular signals. Effector cells in the immune system express inhibitory receptors that serve to limit effector cell expansion and to protect the host from autoreactivity. gp49B is a receptor of unknown function that is expressed on activated mast cells and natural killer (NK) cells and whose cytoplasmic tail endows it with inhibitory potential. To gain insight into the function of gp49B in mice, we disrupted the gp49B gene by homologous recombination. gp49B0 mice were born at expected ratios, were healthy and fertile, and displayed normal long-term survival rates. gp49B0 mice showed no defect in NK or mast cell development. Furthermore, NK and mast cells from the gp49B0mice showed activation properties in vitro similar to those of cells isolated from wild-type mice. Therefore, gp49B is not critical for the development, expansion, and maturation of mast cells and NK cells in vivo. The healthy status of gp49B0 mice makes them suitable for testing the role of gp49B in immune responses to infectious agents.

Structure and Immune Recognition of HLA-B27 Antigens: Implications for Disease Association

HLA-B27 is strongly associated with susceptibility to ankylosing spondylitis and other spondyloarthropathies. Structural analysis of this antigen has revealed the existence of multiple variants, or subtypes, in human populations. The structural microheterogeneity of these subtypes deeply affects allospecific T cell recognition and most of it occurs at an spatial cluster within the peptide binding groove of the molecule. Many polymorphic residues whose combination is unique to HLA-B27 but is conserved among subtypes are clustered in a spatially separated site of the groove from that where most subtype polymorphism occurs. Site-directed mutagenesis and DNA-mediated gene transfer has been used to show that the positions that are polymorphic among subtypes are highly relevant for modulating T cell recognition, so that immunologically silent changes do not occur. These studies have also revealed an extremely high clonotypic diversity in the alloreactive response against HLA-B27. The structural basis for this diversity has been examined by sequencing the clonotypic T cell receptors. The analysis shows a certain bias in V beta gene segment usage, as well as other recurrent structural motives, among T cell receptor beta chains from HLA-B27-specific cytotoxic T cell clones.

Type I transmembrane receptor with inhibitory function in mouse mast cells and NK cells

The MHC class I-specific inhibitory receptors on human and mouse NK cells have surprisingly different structures. The mouse receptors (Ly-49) are type II transmembrane glycoproteins of the C-type lectin family, whereas the human receptors (killer cell inhibitory receptors (KIR)) belong to the Ig superfamily. This difference prompted a search for Ig-like inhibitory receptors in mice. Here we show that gp49, a mouse mast cell protein of unknown function but with sequence similarity to KIR, is expressed in NK cells. The gp49 cytoplasmic tail, containing a sequence related to an inhibitory motif shared by KIR and Ly-49, delivered a strong inhibitory signal in both human and mouse NK cells when substituted for a KIR cytoplasmic tail. These data show that Ig-like receptors with inhibitory properties exist in both species and that mouse mast and NK cells may recognize common inhibitory ligands.

Analysis of HLA-B27-Specific T-Cell Epitopes with Site-Directed Mutants Mimicking HLA-B27 Polymorphism

Studies in several laboratories have demonstrated the existence of at least six subtypes of the HLA-B27 antigen in the human population. They are designated as B*2701 to B*2706 and differ from each other by just a few amino acid changes, that are located in the peptide binding site of the molecule (Lopez de Castro 1989). Because of its location, subtype polymorphism induces significant, sometimes critical, differences in cytotoxic T lymphocyte (CTL) recognition, so that B27 subtypes are, in spite of their structural similarity, functionally distinct alleles.

Structure of HLA-B27 Subtypes: Evolutionary Implications

The HLA-B27 antigen is a well-defined serologic specificity, but at least six structurally different subtypes can be distinguished by biochemical analyses and by cytolytic T lymphocytes. These subtypes are designated as B*2701 to B*2706. The available population studies show an uneven distribution of the various B27 subtypes among the major ethnic groups, B*2705 being predominant in the overall human population (1,2). The structure of these six subtypes has been established by comparative peptide mapping and protein sequence analysis (3, and references therein). Their structural differences are summarized in Table 1. Three structural patterns are readily apparent from this comparison: 1) B*2705, B*2702 and B*2701 differ from each other by two or three substitutions within residues 74 to 81; 2) B*2704 and B*2706 both differ from B27.1 by two amino acid changes at position 77 and 152. B*2706 has two additional substitutions at residues 114 and 116; and 3) B*2703 differs from B*2705 by a single amino acid change. The origin of these structural patterns could be accounted for by three different genetic mechanisms. First, both B*2702 and B*2701 could have arisen by single gene conversion-like events from B*2705. Potential donor sequences for both subtypes are likely to exist in the HLA gene repertoire. Second, the nature of the two non-clustered amino acid differences between B*2705 and B*2704, involving at least two and one base changes respectively, would require a minimum of two genetic steps — either sequential recombination or recombination and point mutation. A further gene conversion-like event would probably be necessary to explain the two additional differences of B*2706. Third, the structure of B*2703 is compatible with its origin from B*2705 by a single point mutation.