Two Structurally Related Rat Ly49 Receptors with Opposing Functions (Ly49 Stimulatory Receptor 5 and Ly49 Inhibitory Receptor 5) Recognize Nonclassical MHC Class Ib-Encoded Target Ligands

Abstract

The Ly49 family of lectin-like receptors in rodents includes both stimulatory and inhibitory members. Although NK alloreactivity in mice is regulated primarily by inhibitory Ly49 receptors, in rats activating Ly49 receptors are equally important. Previous studies have suggested that activating rat Ly49 receptors are triggered by polymorphic ligands encoded within the nonclassical class Ib region of the rat MHC, RT1-CE/N/M, while inhibitory Ly49 receptors bind to widely expressed classical class Ia molecules encoded from the RT1-A region. To further investigate rat Ly49-mediated regulation of NK alloreactivity, we report in this study the identification and characterization of two novel paired Ly49 receptors that we have termed Ly49 inhibitory receptor 5 (Ly49i5) and Ly49 stimulatory receptor 5 (Ly49s5). Using a new mAb (mAb Fly5), we showed that Ly49i5 is an inhibitory receptor that recognizes ligands encoded within the class Ib region of the u and l haplotypes, while the structurally related Ly49s5 is an activating receptor that recognizes class Ib ligands of the u haplotype. Ly49s5 is functionally expressed in the high NK-alloresponder PVG strain, but not in the low alloresponder BN strain, in which it is a pseudogene. Ly49s5 is hence not responsible for the striking anti-u NK alloresponse previously described in BN rats (haplotype n), which results from repeated alloimmunizations with u haplotype cells. The present studies support the notion of a complex regulation of rat NK alloreactivity by activating and inhibitory Ly49 members, which may be highly homologous in the extracellular region and bind similar class Ib-encoded target ligands.

The Novel Inhibitory NKR-P1C Receptor and Ly49s3 Identify Two Complementary, Functionally Distinct NK Cell Subsets in Rats

“…We have recently shown that some inhibitory and activating Ly49 receptors are coexpressed in certain subsets of rat NK cells (11). A striking finding in the present study is that NKR-P1C was expressed mainly by the Ly49-negative fraction of NK cells.…”

“…The precise contribution of inhibitory and activating Ly49 and KIR receptors to NK-mediated alloreactivity remains to be determined. Studies in rats have pointed toward a central role for the activating receptors (10,11). Therefore, it is possible that transplant patients lacking activating KIRs have a reduced risk of rejection of NK-incompatible (KIR/HLA) stem cell grafts in the clinical setting, especially under nonmyeloablative regimens.…”

“…Although inhibitory Ly49 receptors react with both classical (RT1-A) and nonclassical (RT1-CE/ N/M) MHC class I molecules, their activating counterparts mainly recognize nonclassical MHC (8 -11). Furthermore, the activating Ly49 receptors can be instrumental in triggering rat NK alloreactivity (10,11).…”

Two major groups of rat NKR‐P1 receptors can be distinguished based on chromosomal localization, phylogenetic analysis and Clr ligand binding

“…Overlapping specificity of activating and inhibitory pairs is a common theme for several MHC-binding NK receptors including Ly49, KIR, and CD94/NKG2 [28][29][30][31]. The basis for the development of parallel inhibitory and activating NKR-P1 recognition systems for the same ligands is not clear, but the existence of viral Clr decoys [23] suggests that it may be pathogen-driven.…”

Functional characterization of a conserved pair of NKR‐P1 receptors expressed by NK cells and T lymphocytes in liver and gut

“…in the context of inflammation and transplantation. These tasks are controlled by a wide range of receptors with different functions and ligands, such as Ly49 receptors, which recognize both classical and nonclassical MHC class I molecules, as well as MHC Correspondence: Dr. Lise Kveberg e-mail: lise.kveberg@rr-research.no class I-like virally encoded ligands [1][2][3]. Another family of MHCbinding receptors is the conserved NKG2/CD94 heterodimers, which bind the nonclassical MHC molecule HLA-E in human [4], its rodent homologue Qa-1 b in the mouse [5,6] and RT.BM1 in the rat [7].…”