Stimulatory catfish leukocyte immune-type receptors (IpLITRs) demonstrate a unique ability to associate with adaptor signaling proteins and participate in the formation of homo- and heterodimers.

Channel catfish leukocyte immune-type receptors (IpLITRs) are immunoglobulin superfamily (IgSF) members believed to play a role in the control and coordination of cellular immune responses. Some IpLITR-types encode a transmembrane (TM) region containing a single positive charged lysine (K) residue, which is a key feature of stimulatory immune receptors that associate with immunoreceptor tyrosine-based activation motif (ITAM)-containing adaptor proteins. In this study we focused on identifying the signaling adaptor molecules recruited by putative stimulatory IpLITRs as a first step towards elucidating their ability to regulate catfish immune cell effector functions. Using HEK 293T cells co-transfected with epitope-tagged catfish proteins, we demonstrate that IpLITRs associated with the IpFcRgamma, IpFcRgamma-L, and IpCD3zeta-L adaptors, which all encode a negative charged aspartic acid (D) residue within their TM regions. Association of IpLITRs with IpFcRgamma and IpFcRgamma-L also enhanced cell surface expression of the receptor, which was not observed after co-transfections with IpCD3zeta-L, IpDAP12, or IpDAP10. Mutating the lysine residue (at amino acid position 199) within the TM region of IpLITR 2.6b to alanine (A(199)) did not prevent the association with IpFcRgamma-L and only slightly reduced receptor expression levels on the cell surface. Surprisingly, this mutation also facilitated IpLITR 2.6b association with IpDAP12 that correlated with an enhanced expression of the receptor. Conversely, an aspartic acid (D(30)) to A(30) switch within the IpFcRgamma-L TM region completely abrogated its assembly with the receptor and inhibited the IpFcRgamma-L induced surface expression of IpLITR 2.6b. In addition, co-transfections and immunoprecipitation of single (i.e. N-terminal HA) and double (i.e. N-terminal HA and C-terminal 3xFLAG) epitope-tagged stimulatory IpLITR-types revealed that these immune receptors formed non-covalent homo- and heterodimers through interaction(s) likely mediated by their extracellular immunoglobulin (Ig)-like domains. Combined with their unique association with adaptor proteins, dimerization may have profound effects on IpLITR-mediated regulation of teleost immune responses by influencing their signaling potential and/or ligand-binding properties.