Natural killer (NK) cells are a unique group of lymphocytes involved in the surveillance and killing of foreign or infected cells through a mechanism involving recognition of HLA molecules by an extremely diverse set of receptors on the NK cell surface. The killer immunoglobulin-like receptor genes (KIRs) map to chromosome 19q13.4 along with other related genes. Some of these genes encode molecules that recognize HLA-C ligands, whereas others bind HLA-A and -B molecules. In contrast to cytotoxic T lymphocytes (CTL) recognition of peptide as presented by class I, NK cells destroy targets that lack expression of class I, and are inhibited by recognition of class I on the cell surface. NK cells play an important role in defense against virally infected cells and tumor cells that downregulate class I molecule expression, thereby avoiding recognition by CTL. For example, HIV-1 downregulates cell surface expression of HLA-A and -B molecules (but not -C) and one mechanism to explain rapid progression to AIDS in individuals with certain HLA types may involve alteration of NK cell activity (see Project number Z01 BC 10269-05 LGD). Evidence for downregulation of class I by human papilloma virus (HPV) has also been reported. Haplotypes of the KIR gene complex are extremely diverse, varying in the number and type of genes present, and some of the genes are polymorphic. We have developed a molecular typing technique to determine the presence or absence of each of the 15 KIR genes in order to study their potential effects on autoimmune (psoriatic arthritis [PsA], ankylosing spondylitis and multiple sclerosis), infectious diseases (those associated with HIV-1, hepatitis C virus, and hepatitis B virus), and cancer (colorectal and cervical). Initial studies have indicated the need to define diversity of KIR haplotypes in order to conduct rigorous disease association studies with this locus. Thus, in order to define KIR haplotypes and their frequencies we have typed 59 CEPH families for presence or absence of the KIR genes, representing nearly 1,400 KIR haplotypes (about 370 independent chromosomes and 1,000 meioses). Some KIR genes are found on almost all KIR haplotypes, so when one parent has two copies of a gene (one on each chromosome) and the other parent has one or two, all offspring are positive for the gene and it is not possible to determine whether one chromosome may be missing that gene locus. In some cases, it may be possible to resolve this problem by typing single nucleotide polymorphisms present in that gene. Thus, we have designed a sequence-based typing system for one of the common KIR genes, KIR2DL4, and an SSOP system for another common gene, KIR3DL1. KIR2DL4 allelic typing in the families is nearly complete and KIR3DL1 typing will be started shortly. It is clear from the data we have collected thus far that KIR haplotypes are extremely complex, and the most obvious mechanism for haplotypic variation appears to be unequal crossing over. However, the family data do not indicate a hotspot for recombination within the KIR gene complex. Ongoing typing and analyses of these haplotypes are underway. NK cell activity is partially controlled through interactions between KIR on NK cells and their respective HLA class I ligands. Given the functional relationship of these molecules, which are both encoded by highly polymorphic, unlinked regions of the genome, it is possible that diseases influenced by certain HLA alleles may also be affected by the genes encoding the corresponding KIR molecules. Independent segregation of HLA and KIR genes along with KIR specificity for particular HLA allotypes raises the possibility that any given individual may express KIR molecules for which no ligand is present. We have performed KIR genotyping in a cohort of individuals with psoriatic arthritis, a disease that is strongly associated with HLA. The data indicate that subjects with activating KIR2DS1 and/or KIR2DS2 genes are susceptible to developing psoriatic arthritis, but only when HLA ligands for their homologous inhibitory receptors, KIR2DL1 and KIR2DL2/3, are missing. Absence of ligands for inhibitory KIRs could potentially lower the threshold for NK (and/or T) cell activation mediated through activating receptors, thereby contributing to the pathogenesis of psoriatic arthritis. We have recently completed typing of more than 500 individuals from two cohorts of cervical cancer (Eastern US and Costa Rica) and are in the process of typing a third group. Preliminary analysis of the combined data thus far indicates that the inhibitory receptors KIR3DL1 and KIR2DL1 in the presence of their HLA ligands (Bw4 and Cw group 2 respectively) were protective (3DL1+Bw4: OR = 0.60, CI = 0.41-0.88; 2DL1+Cw group 2: OR = 0.67, CI = 0.46-0.97). These associations were also seen when the groups were analyzed separately, although the results were less significant, probably as a result of diminished sample size. KIR typing has also begun on a cohort of approximately 500 colorectal cancers from Slovenia and we are in the process of acquiring more control samples. The data thus far indicate that the activating receptors KIR2DS1 and KIR3DS1 show a significant association with disease. We have found a general trend for an association of activating KIRs with a favorable outcome in infectious diseases, where the presence of KIR3DS1 appears protective in HIV disease progression, and potentially in Hepatitis C virus clearance. Conversely, in autoimmune disease and cancer, the activating KIRs tend to be associated with an unfavorable outcome. A complete, thorough analysis of each of the diseases mentioned herein is planned for this next fiscal year.
