The signaling lymphocyte activation molecule (SLAM) family of immune cell receptors is closely related to the CD2 family of the immunoglobulin (Ig) superfamily of molecules. The SLAM family currently includes nine members named SLAM, CD48, CD229, 2B4, CD84, NTB-A, CRACC, BLAME, and CD2F-10 (Figure 1).1-3 In general, SLAM molecules possess two to four extracellular Ig domains, a transmembrane segment, and an intracellular tyrosine-rich region. The molecules are differentially expressed on a variety of immune cell types. Several are self ligands and SLAM has been identified as the human measles virus receptor (Table 1).1-3 Several small SH2- containing adaptor proteins are known to associate with the intracellular domains of SLAM family members. For example, in T and NK cells, activated SLAM family receptors become tyrosine phosphorylated and recruit the adaptor known as SAP (SLAM-associated protein) and subsequently the Src kinase Fyn, the ensuing signal transduction cascade influences the outcome of T cell–antigen presenting cell and NK cell–target cell interactions.1,2,4,5
|Figure 1. Schematic of the phylogenetic relationships of the SLAM family members. [Note: figure adapted from Fraser, C.C. et al (2002) Immunogenetics 53:843.]|
The functional significance of SLAM family members is emphasized by their involvement in X-linked lymphoproliferative (XLP) disease, also known as Purtilo’s syndrome and Duncan’s disease. XLP disease is an inherited immunodeficiency usually triggered by Epstein-Barr virus (EBV) infection. Most EBV-infected XLP patients mount an inappropriate immune response, which results in infectious mononucleosis, organ failure, and death. Surviving individuals or those not infected with EBV typically are afflicted with malignant non-Hodgkin’s lymphomas or other lymphoproliferative disorders. Evidence suggests that a significant proportion of XLP disease cases are attributable to the nearly 50 different loss-of-function mutations so far described in the gene encoding SAP.1,2,4,5
Several laboratories have developed SAP knockout mouse models to more closely examine the effects of SAP mutations at the cellular level. The defects observed thus far in the SAP knockouts suggest significant T cell dysregulation. The number of CD8+ and CD4+ T cells is increased, CD4+ T cells display a decreased ability to differentiate into Th2 cells resulting in an overall Th1 skewing of the immune response, and the number of memory B and plasma cells is decreased likely due to decreased CD4+ T cell help.6-8 More recently, Chung et al. reported that SAP knockout mice, as well as XLP patients, completely lack CD1d-restricted NKT cells.9 This effect may be related to the observation that Fyn mutant and Fyn knockout mice also fail to develop CD1d-restricted NKT cells.10,11
|Table 1. The human SLAM family. *Systematic nomenclature is included as described by the HUGO Gene Nomenclature Committee at http://www.gene.ucl.ac.uk/nomenclature/.|
The details of the relationship between altered T cell responses and lack of NKT cell populations, and how these perturbations translate into the various clinical manifestations of XLP, are not known. However, one clue is provided by a recent study that describes NKT cells as having a tempering effect on EBV-induced T cell proliferation, which may be a means of containing T cell responses and thereby preventing destructive immune system over-reactions.12 This fine balance between enough and too much appears to be the main function of a variety of regulatory immune cell receptors, including SLAM family molecules.