Charles Gullo

SAP controls T cell responses to virus and terminal differentiation of TH2 cells.

SH2D1A, which encodes signaling lymphocyte activation molecule (SLAM)–associated protein (SAP), is altered in patients with X-linked lymphoproliferative disease (XLP), a primary immunodeficiency. SAP-deficient mice infected with lymphocytic choriomeningitis virus had greatly increased numbers of CD8+ and CD4+ interferon-γ–producing spleen and liver cells compared to wild-type mice. The immune responses of SAP-deficient mice to infection with Leishmania major together with in vitro studies showed that activated SAP-deficient T cells had an impaired ability to differentiate into T helper 2 cells. The aberrant immune responses in SAP-deficient mice show that SAP controls several distinct key T cell signal transduction pathways, which explains in part the complexity of the XLP phenotypes.

The Cell Surface Receptor SLAM Controls T Cell and Macrophage Functions

Signaling lymphocyte activation molecule (SLAM), a glycoprotein expressed on activated lymphocytes and antigen-presenting cells, has been shown to be a coregulator of antigen-driven T cell responses and is one of the two receptors for measles virus. Here we show that T cell receptor–induced interleukin (IL)-4 secretion by SLAM−/− CD4+ cells is down-regulated, whereas interferon γ production by CD4+ T cells is only slightly up-regulated. Although SLAM controls production of IL-12, tumor necrosis factor, and nitric oxide in response to lipopolysaccharide (LPS) by macrophages, SLAM does not regulate phagocytosis and responses to peptidoglycan or CpG. Thus, SLAM acts as a coreceptor that regulates signals transduced by the major LPS receptor Toll-like receptor 4 on the surface of mouse macrophages. A defective macrophage function resulted in an inability of SLAM−/− C57Bl/6 mice to remove the parasite Leishmania major. We conclude that the coreceptor SLAM plays a central role at the interface of acquired and innate immune responses.

Structural basis for the interaction of the free SH2 domain EAT-2 with SLAM receptors in hematopoietic cells

The T and natural killer (NK) cell‐specific gene SAP (SH2D1A) encodes a ‘free SH2 domain’ that binds a specific tyrosine motif in the cytoplasmic tail of SLAM (CD150) and related cell surface proteins. Mutations in SH2D1A cause the X‐linked lymphoproliferative disease, a primary immunodeficiency. Here we report that a second gene encoding a free SH2 domain, EAT‐2, is expressed in macrophages and B lympho cytes. The EAT‐2 structure in complex with a phosphotyrosine peptide containing a sequence motif with Tyr281 of the cytoplasmic tail of CD150 is very similar to the structure of SH2D1A complexed with the same peptide. This explains the high affinity of EAT‐2 for the pTyr motif in the cytoplasmic tail of CD150 but, unlike SH2D1A, EAT‐2 does not bind to non‐phosphorylated CD150. EAT‐2 binds to the phosphorylated receptors CD84, CD150, CD229 and CD244, and acts as a natural inhibitor, which interferes with the recruitment of the tyrosine phosphatase SHP‐2. We conclude that EAT‐2 plays a role in controlling signal transduction through at least four receptors expressed on the surface of professional antigen‐presenting cells.

Expression of the SH2D1A gene is regulated by a combination of transcriptional and post-transcriptional mechanisms

The SH2D1A gene, which is altered or deleted in patients with X‐linked lymphoproliferative disease, encodes the small protein SAP (for SLAM‐associated protein) that is expressed in T and NK cells. A 22‐bp fragment in close proximity to an initiator‐like site was defined as the basal promoter of mouse SH2D1A, and a highly homologous 33‐bp segment was defined as the human basal promoter. When an Ets consensus site was mutated, no reporter activity was detectable. Gel mobility supershift assays revealed that the two transcription factors Ets‐1 and Ets‐2 bind to the human and mouse sequences. The involvement of Ets‐1 and Ets‐2 in expression of SH2D1A was functionally confirmed by overexpression studies of their dominant‐negative forms. We also found that SH2D1A mRNA decays very rapidly in mouse T cells, and its 3′ untranslated region (UTR) has RNA‐destabilizing activity in transfection studies with reporter/3′ UTR constructs. As judged by RNA‐gel mobility shift assays, this rapid degradation of SH2D1A mRNA was due to a balance in binding of the factors AUF1 and HuR to its 3′ UTR. Although the SH2D1A mRNA level decreased upon triggering of the T cell receptor (TCR), the RNA degradation rate itself was not altered by TCR engagement.