Biology of inhibitory immune receptors : Studies on CD200R and LAIR-1

Abstract

The primary function of the immune system is to protect the body against invading pathogens. However, the mechanisms it employs to kill pathogens can also kill cells of the body. To prevent this all immune cells express inhibitory receptors. These receptors can function by preventing unnecessary activation or terminating immune reactions when the causative agent is removed. Many inhibitory immune receptors relay their inhibitory signal into the cell via inhibitory motifs (ITIMs). When the inhibitory receptor is bound by its ligand the tyrosine residues in the ITIM are phosphorylated and recruit phosphatases. These phosphatases can dephosphorylate activating molecules, thereby inhibiting cellular activation. In this thesis, we investigate the mechanism of action of two different inhibitory immune receptors: LAIR-1 and CD200R. LAIR-1 is an inhibitory receptor expressed on almost all immune cells that contains two ITIMs in its intracellular domain. LAIR-1 is known to recruit phosphatases SHP-1 and SHP-2 after phosphorylation. However, here we report that LAIR-1 has phosphatase-independent activity and show that the C-terminal Src family kinase Csk interacts with LAIR-1 and is a potential mediator of its inhibitory function. CD200R is an atypical inhibitory immune receptor that does not contain ITIMs, but does contain three single tyrosines in its intracellular domain. We report that CD200R only has full inhibitory capacity when all three intracellular tyrosines are present. In addition, we found association of Dok-1 and Shc to CD200R using a yeast tri hybrid system. Thus, we conclude that not all immune inhibition is relayed by inhibitory receptors signalling through ITIMs and phosphatases. CD200R is known to be an important inhibitor of myeloid cell function. Mice deficient for the ligand of CD200R, CD200, have enhanced activation of the myeloid cells expressing CD200R and show increased susceptibility to the induction of auto-immune diseases, due to reduced inhibition. Dok-1 and Dok-2 both are reported to associate to CD200R. Since Dok-1/Dok-2 double knockout mice develop myeloid leukaemia, we investigated whether CD200R signalling is involved in leukaemia development. We report that CD200-/- mice have normal numbers of myeloid progenitor and stem cells and that these cells have normal proliferative capacity. In addition, CD200-/- mice do not develop leukaemia even at old age. Thus, we conclude that CD200R signalling is not involved in leukaemia development in Dok-1/Dok-2 double knockout mice. Previously, CD200R expression was mainly reported on myeloid cells. However, here we demonstrate that CD200R is also differentially expressed on subsets of both human and mouse T and B lymphocytes. To investigate the role of CD200R signalling in the immune response against a virus, we infected CD200-/- mice with influenza. Surprisingly, CD200-/- mice develop severe disease upon influenza infection, whereas control wild type mice develop only mild symptoms. After depletion of almost all CD4+ and CD8+ T cells before influenza infection, CD200-/- mice did not develop severe disease, indicating T cells may play a role in the development of the severe disease of CD200-/- mice after influenza infection. These data indicate that CD200 is an important host factor in the determination of the outcome of influenza infections.