Proliferation and differentiation of alloselective NK cells after alloimmunization-evidence for an adaptive NK response

Identification of MHC Class Ib Ligands for Stimulatory and Inhibitory Ly49 Receptors and Induction of Potent NK Cell Alloresponses in Rats

Early studies indicate that rats may have a repertoire of MHC class Ib-reactive Ly49 stimulatory receptors capable of mounting memory-like NK cell alloresponses. In this article, we provide molecular and functional evidence for this assumption. Pairs of Ly49 receptors with sequence similarities in the lectin-like domains, but with opposing signaling functions, showed specificity for ligands with class Ia-like structural features encoded from the first telomeric MHC class Ib gene cluster, RT1-CE, which is syntenic with the H2-D/H2-L/H2-Q cluster in mice. The activating Ly49s4 receptor and its inhibitory counterparts, Ly49i4 and Ly49i3, reacted with all allelic variants of RT1-U, whereas Ly49s5 and Ly49i5 were specific for RT1-E^u NK cell cytolytic responses were predictably activated and inhibited, and potent in vivo NK alloresponses were induced by repeated MHC class Ib alloimmunizations. Additional Ly49-class Ib interactions, including RT1-C^l with the Ly49s4/Ly49i4/Ly49i3 group of receptors, were characterized using overexpressed receptor/ligand pairs, in vitro functional assays, and limited mutational analyses. Obvious, as well as subtle, Ly49-class Ib interactions led to ligand-induced receptor calibration and NK subset expansions in vivo. Together, these studies suggest that in vivo NK alloresponses are controlled by pleomorphic Ly49-class Ib interactions, some of which may not be easily detectable in vitro.

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

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.

Activating and inhibitory Ly49 receptors modulate NK cell chemotaxis to CXC chemokine ligand (CXCL) 10 and CXCL12

NK cells can migrate into sites of inflammatory responses or malignancies in response to chemokines. Target killing by rodent NK cells is restricted by opposing signals from inhibitory and activating Ly49 receptors. The rat NK leukemic cell line RNK16 constitutively expresses functional receptors for the inflammatory chemokine CXC chemokine ligand (CXCL)10 (CXCR3) and the homeostatic chemokine CXCL12 (CXCR4). RNK-16 cells transfected with either the activating Ly49D receptor or the inhibitory Ly49A receptor were used to examine the effects of NK receptor ligation on CXCL10- and CXCL12-mediated chemotaxis. Ligation of Ly49A, either with Abs or its MHC class I ligand H2-D(d), led to a decrease in chemotactic responses to either CXCL10 or CXCL12. In contrast, Ly49D ligation with Abs or H2-D(d) led to an increase in migration toward CXCL10, but a decrease in chemotaxis toward CXCL12. Ly49-dependent effects on RNK-16 chemotaxis were not the result of surface modulation of CXCR3 or CXCR4 as demonstrated by flow cytometry. A mutation of the Src homology phosphatase-1 binding motif in Ly49A completely abrogated Ly49-dependent effects on both CXCL10 and CXCL12 chemotaxis, suggesting a role for Src homology phosphatase-1 in Ly49A/chemokine receptor cross-talk. Ly49D-transfected cells were pretreated with the Syk kinase inhibitor Piceatannol before ligation, which abrogated the previously observed changes in migration toward CXCL10 and CXCL12. Piceatannol also abrogated Ly49A-dependent inhibition of chemotaxis toward CXCL10, but not CXCL12. Collectively, these data suggest that Ly49 receptors can influence NK cell chemotaxis within sites of inflammation or tumor growth upon interaction with target cells.

Self-MHC class Ia (RT1-A(n)) protects cells co-expressing the activatory allogeneic MHC class Ib molecule (RT1-E(u)) from NK lysis

We have previously shown activation of NK cells via recognition of an allogeneic, non-classical MHC class I molecule, RT1-E(u). In this study we investigated whether a self-MHC class I molecule could protect the allogeneic targets from being recognized and killed by the alloreactive NK (allo NK) cells. NK cells from BN (RT1 n) rats, primed in vivo by immunization with RT1(u)-expressing cells, manifested cytolytic activity against RT1(u)- as well as RT1(u/lv1)-expressing targets, but not against RT1(u/n)-expressing targets. The absence of cytolytic activity against semiallogeneic targets, i.e. targets expressing self-allotypes, was also valid for allo NK cells from alloimmunized F344 (RT1 (lv1)) rats. To analyze the ability of a distinct MHC class I molecule to protect target cells from NK lysis, Rat2 cells transfected with the activating allogeneic MHC class Ib, RT1-E(u) molecule were also transfected with the self-MHC class Ia, RT1-A1(n) molecule. The allo NK cells from BN rats immunized with RT1(u)-expressing cells were cytolytic against Rat2 transfected with the RT1-E(u) molecule. However, the allo NK cells manifested no cytolytic activity against double-transfected Rat2 cells, expressing the RT1-E(u) as well as the RT1-A1(n) molecule. We conclude that expression of a self-MHC class Ia (RT1-A) molecule protects targets from allo NK killing. Furthermore, the NK inhibition via recognition of the self-MHC class Ia molecule dominates over the activation via recognition of the allogeneic MHC class Ib molecule, RT1-E.