During human CMV infection, there is a preferential expansion of natural killer (NK) cells expressing the activating CD94-NKG2C receptor complex, implicating this receptor in the recognition of CMV-infected cells. We hypothesized that NK cells expanded in response to pathogens will be marked by expression of CD57, a carbohydrate antigen expressed on highly mature cells within the CD56

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Natural killer (NK) cells play a critical role in the innate immune response against cytomegalovirus (CMV) infections. Although CMV encodes several gene products committed to evasion of adaptive immunity, viral modulation of NK cell activity is only beginning to be appreciated. A previous study demonstrated that the mouse CMV m152-encoded gp40 glycoprotein diminished expression of ligands for the activating NK cell receptor NKG2D on the surface of virus-infected cells. Here we have defined the precise ligands that are affected and have directly implicated NKG2D in immune responses to CMV infection in vitro and in vivo. Murine CMV (MCMV) infection potently induced transcription of all five known retinoic acid early inducible 1 (RAE-1) genes (RAE-1α, RAE-1β, RAE-1δ, RAE-1ɛ, and RAE-1γ), but not H-60. gp40 specifically down-regulated the cell surface expression of all RAE-1 proteins, but not H-60, and diminished NK cell interferon γ production against CMV-infected cells. Consistent with previous findings, a m152 deletion mutant virus (Δm152) was less virulent in vivo than the wild-type Smith strain of MCMV. Treatment of BALB/c mice with a neutralizing anti-NKG2D antibody before infection increased titers of Δm152 virus in the spleen and liver to levels seen with wild-type virus. These experiments demonstrate that gp40 impairs NK cell recognition of virus-infected cells through disrupting the RAE-1–NKG2D interaction.

SummaryWe have previously described the isolation of a cDNA clone, designated NKG2, that was expressed in all natural killer (NK) cells tested but not in T or B cells . In the present communication, the original isolate, when used to probe a cDNA library prepared from a CD3 -NK cell clone, was found to crosshybridize with a family of transcripts that fell into four distinct groups designated NKG2-A, -B, -C, and -D. Full-length cDNA sequences were determined for each group, and the DNA and inferred peptide sequences were analyzed. All four transcripts encode type II membrane proteins of 215-233 amino acids. NKG2-A and -B peptides appear to be alternative splicing products of a single gene. NKG2-C is highly homologous with group A, having 94% homology in the external (COOH-terminal) domain and 56% homology throughout the internal and transmembrane regions . NKG2-D is distantly but significantly related (21% amino acid homology) to the first three groups . Therefore, NKG2-A, -C, and -D appear to be encoded by distinct genes within a family of NK cell-specific genes . Peptide sequence homology searches demonstrate that the NKG2 peptides are members of a supergene family that includes several other type II membrane proteins . This family is characterized by the presence of a C-type animal lectin domain, and several of its members have demonstrated transmembrane signaling capability. N K cells appear to be closely related to T cells based upon the common expression of a number of cell type-specific transcripts and cell surface antigens (1, 2). NK cells and T cells also share expression of perforin and serine proteases, components of the cytolytic machinery that are found almost exclusively in lytic cells . However, the TCR/CD3 complex responsible for triggering lytic activity in T cells is absent from NK cells, and a corresponding structure that recognizes the NK target antigen and triggers NK lytic activity has not been identified . Elucidation of these structures would greatly facilitate an understanding of the in vivo function of NK cells.We initiated a project to identify, through the use of differential hybridization and cDNA subtraction methodology, genes that are expressed preferentially in NK cells, with particular emphasis on those encoding membrane-bound proteins (1). In an earlier report, we identified 12 groups of crosshybridizing cDNA clones that were expressed in NK cells but not in an EBV transformed B cell line. One of these groups, NK group 2 (NKG2), was expressed only in NK cells among the panel of cells that we examined . In the current paper, we report that NKG2 is comprised of a group of related transcripts that encode a series of type 11 membrane proteins. Materials and MethodsA cDNA library was prepared from B22, a CD3-, CD16 -, CD56 + human NK clone that lyres K562 (1) . The library was probed with the previously described NKG2 isolate (1) to find additional cDNA clones. DNA sequences of NKG2-A, -B, and -C were determined on templates derived from the original X-Gem cDNA clones via the asymmetr...

Lectin-like transcript-1 (LLT1) (also named osteoclast inhibitory lectin or CLEC2D) is a ligand for the human NKR-P1A (CD161) receptor, present on NK cells and T cells. To further understand the physiological relevance of this interaction, we developed mAbs against LLT1, characterized the expression pattern of LLT1, and explored the functional consequence of LLT1 engagement of the NKR-P1A receptor on NK cells and T cells. LLT1 is expressed on TLR-activated plasmacytoid dendritic, TLR-activated monocyte-derived dendritic cells, and on B cells stimulated through TLR9, surface Ig, or CD40. Interactions between NKR-P1A on NK cells and LLT1 on target cells inhibit NK cell-mediated cytotoxicity and cytokine production and can inhibit TNF-α production by TCR-activated NKR-P1A+ CD8+ T cells. In contrast, NKR-P1A failed to inhibit or augment the TCR-dependent activation of NKR-P1A-bearing CD4+ T cells. Expression of LLT1 on activated dendritic cells and B cells suggests that it might regulate the cross-talk between NK cells and APCs.