the signaling lymphocyte activation molecule ( slam ) family is a group of six immunoglobulin - like receptors named slam ( or cd150 ) , 2b4 ( or cd244 ) , ly-9 ( or cd229 ) , nk - t - b antigen ( ntb - a ) ( or ly-108 ) , cd84 and cd2-like receptor activating cytotoxic t cells ( cracc ) ( 1 , 2 ) .
these receptors are differentially expressed in various immune cell types and are close relatives of the cd2 family .
some , including slam , ntb - a , and cracc , are self - ligands .
at least one , 2b4 , is implicated in heterotypic interactions with another immune cell receptor , cd48 .
slam is also the immune cell - specific receptor for measles virus in humans . until now
data from experiments in which cells were treated with antibodies or ligands presumed to stimulate slam - related receptors , although at times contradictory , suggested that slam - related receptors modulate the immune response .
the notion that slam family receptors are immunoregulatory was supported by the observation that they associate , by way of cytoplasmic tyrosine - based motifs , with the small sh2 domain  containing adaptor sap ( 2 , 3 ) .
sap is expressed in t cells and nk cells and is mutated in x - linked lymphoproliferative ( xlp ) disease , a human immune disorder characterized by a dysregulated immune response to epstein - barr virus ( ebv ) infection . since sap allows slam - related receptors to associate with the src - related protein tyrosine kinase fyn and to mediate protein tyrosine phosphorylation signals ( 46 ) , it was postulated that xlp may be the result of inadequate slam family receptor functions in cells normally expressing sap . in agreement with this ,
defects in cd4 and cd8 t cell functions were described in sap - deficient mice ( 79 ) .
the impact of sap deficiency on nk cell functions in mice remains to be examined . in spite of the mounting evidence that slam - related receptors utilize sap in their signaling mechanism
, some data suggest that they can mediate functions in the absence of sap ( 1 , 2 ) .
first , slam - related receptors are abundantly expressed in cell types that do not express sap , including macrophages and dcs .
these cell types and nk cells contain a sap homologue termed eat-2 , which can associate with slam - related receptors ( 1 , 2 ) .
whereas the role of eat-2 in cell signaling is not known , eat-2 lacks the motif that enables sap to activate fyn . on that basis
second , antibody - mediated engagement of 2b4 on nk cells derived from xlp patients was reported by one group to inhibit , rather than activate , nk cell  mediated killing ( 10 ) .
thus , 2b4 may mediate an alternate signal , leading to nk cell inhibition , in the absence of sap . whether this signal is due to eat-2
third , slam - related receptors were reported to associate with other molecules ( such as the protein tyrosine phosphatase shp-2 ) and to mediate certain types of biochemical signals ( such as akt activation ) in the absence of sap .
nevertheless , these data were primarily generated through overexpression studies in nonimmune cells , and their physiological significance is unclear .
slam is a homotypic receptor found on t cells , b cells , macrophages , and dcs ( 1 , 2 ) .
a possible role for slam was first characterized in human t cells ( 11 ) .
engagement of slam by anti - slam mabs evoked tcr - independent cd4 t cell proliferation and ifn- secretion , but not il-4 production , by previously activated human t cells .
this led to the idea that slam was a novel activating receptor on t cells that favored th1 cytokine production .
subsequent studies with mouse t cells failed to confirm that antibody - mediated slam ligation was sufficient to trigger t cell activation ( 12 ) .
nonetheless , slam - specific antibodies moderately enhanced mouse t cell proliferation and ifn- secretion in response to low doses of anti - cd3 mab .
this effect was also observed in t cells derived from sap - deficient mice , indicating that these responses were sap independent ( fig .
1 a ; reference 13 ) . however , a significant limitation of these studies was that it was not known whether the anti - slam antibodies were triggering or blocking the function of slam . functions of slam family receptors are regulated by differential expression of sap - related adaptors . ( a ) model of slam function . in t cells expressing sap ,
slam  slam interactions trigger selective up - regulation of il-4 secretion in response to antigen receptor stimulation . although the downstream targets responsible for this activity are not yet understood , they may involve previously described slam - sap effectors , such as fyn , ship-1 , dok-1 , dok-2 , shc , and ras - gap ( 5 ) . in the absence of sap
, slam could mediate a different signal , which may lead to down - regulation of ifn- secretion .
2b4cd48 interactions enhance cytotoxicity and secretion of ifn-. this response may involve known targets of 2b4-sap signaling , such as vav-1 , c - cbl , and ship-1 ( 22 , 23 ) . in the presence of eat-2 , 2b4 could induce nk cell inhibition . in the absence of both sap and eat-2 ,
2b4 could trigger yet another signal that inhibits nk cell functions . in this issue , wang et al .
they found that slam cd4 t cells had a severe defect in tcr - mediated production of il-4 in vitro .
although the impact of slam deficiency on il-4 provides evidence for the importance of slam in immunity , the lack of a substantial effect on ifn- is intriguing .
most likely , there is redundancy with other receptors having ifn-inducing functions that compensate for the absence of slam . in support of this
, it is known that t cells express other slam family receptors , including ly-9 and ntb - a .
the small increase in ifn- production observed in slam - deficient t cells also suggests that slam signaling may inhibit rather than augment ifn- release .
considering the marked defect in il-4 production in slam - deficient cd4 t cells , the lack of an effect of anti - slam antibodies on il-4 release is perplexing .
one interpretation is that anti - slam antibodies impinge on only part of the function of slam , either by affecting selective slam  slam interactions or by altering the surface distribution of slam .
consequently , anti - slam antibodies may influence the coupling of slam to pathways regulating ifn- without interfering with those controlling il-4
. a clearer understanding of these contradictory observations will likely be provided by a better biophysical and biochemical analysis of slam and slam - associated signaling pathways in t cells .
similar to slam - deficient t cells , t cells from sap mice have markedly reduced il-4 production ( 7 , 8) .
this similarity implies that slam regulates il-4 release through its interaction with sap ( fig .
moreover , it suggests that the defect in il-4 production in sap t cells is largely due to a block in slam receptor signal transduction . however , the il-4 defect was more drastic in t cells from sap animals than slam animals , raising the possibility that other slam family receptors regulate il-4 through a sap - dependent pathway . in spite of the clear link between slam and il-4
, it must be emphasized that the defects in cd4 t cell help seen in vivo in sap - deficient mice do not seem to be due to a simple lack of il-4 production ( 9 ) .
other as yet unidentified th2-related alterations appear to be implicated . hence , to establish the relative importance of slam dysfunctions in sap - dependent immune defects it will be crucial to assess the extent of functional t cell abnormalities in vivo in slam mice .
slam - deficient macrophages were found to exhibit abnormal functions in vitro and in vivo ( 14 ) .
production of il-12 , tnf- , and nitric oxide ( no ) in response to lps , with or without ifn- , was diminished in slam peritoneal macrophages .
in contrast , il-12 and tnf- secretion triggered by toll - like receptor agonists , such as cpg dna and peptidoglycan , and the capacity of macrophages to present antigen to t cells were normal .
in vivo studies showed that the ability of c57bl/6 mice to clear leishmania major infection was compromised in the absence of slam . in this mouse strain , healing of leishmania major lesions is known to rely heavily on macrophage functions , including il-12 , tnf- , and no production .
the mechanism by which slam regulates macrophage responses to lps was not established . since stimulation with anti - slam antibodies was sufficient to trigger il-12 secretion and inhibit il-6 release by inflammatory macrophages , it is plausible that slam  slam interactions between neighboring macrophages induce a signal that influences cytokine release in response to lps .
however , given the similarities between lps , cpg , and peptidoglycan signaling , it is not clear why the lack of slam expression would only affect lps - triggered responses . moreover , the nature of the slam signals regulating il-12 and il-6 synthesis in macrophages remains to be determined .
macrophages do not express sap but do express eat-2 ( 2 , 3 ) . as discussed above , eat-2 may or may not play a role analogous to that of sap in slam family receptor signaling . it is also conceivable that slam regulates macrophage function independently of sap - related adaptors .
the characterization of slam - induced biochemical signals in macrophages and of the role of eat-2 in these cells will help clarify these issues .
2b4 is found on nk cells , cd8 t cells ,  t cells , and monocytes ( 1 , 2 ) . its ligand is cd48 , a glycophosphatidylinositol ( gpi)-linked receptor broadly expressed on immune cells .
previous studies showed that engagement of 2b4 on human or mouse nk cells by anti-2b4 antibodies results in production of ifn- ( 1 , 2 ) . furthermore ,
anti-2b4 antibodies trigger nk cell  mediated killing in reverse antibody - dependent cellular cytotoxicity assays .
these effects were observed to be seriously compromised in xlp - derived nk cells , implying that they rely on the ability of 2b4 to associate with sap in humans .
engagement of 2b4 expressed on cd8 t cells by cd48 was also reported to enhance tcr - mediated cytotoxicity ( 15 ) .
thus , together these results led to the belief that 2b4 is an activating receptor , which augments the responsiveness of nk cells and cd8 t cells . in this issue , lee et al .
unexpectedly , they observed that cell - mediated cytotoxicity and ifn- secretion was enhanced rather than diminished in 2b4 nk cells .
the ability of nk cells to eliminate tumor cells in an intraperitoneal tumor clearance assay was also increased .
these altered functions were fully dependent on expression of cd48 on target cells and were blocked by addition of anti - cd48 antibodies .
they were also restored in 2b4-deficient nk cells by retrovirus - mediated expression of the long form of 2b4 , but not the short form , which has fewer tyrosine - based motifs in its cytoplasmic domain .
these unanticipated results imply that 2b4 is primarily an inhibitory receptor in mouse nk cells and suggest that the ability of anti-2b4 antibodies to trigger nk cell functions in mice , and possibly humans , may reflect their capacity to block 2b4cd48 interactions .
however , there are still compelling data indicating that expression of cd48 on target cells can enhance the capacity of human nk cells to kill ( 17 ) .
additionally , one group reported that target cell killing was defective in nk cells from an xlp patient even in the absence of additional anti-2b4 antibodies ( 18 ) .
there is also convincing evidence that the presence of cd48 on 2b4 cd8 mouse t cells enhances antigen - induced t cell cytotoxicity ( 15 ) .
therefore , 2b4 may be able to act either as an inhibitory receptor or as an activating receptor ( fig .
one scenario is that 2b4 mediates these opposite effects in different subsets of nk cells . in support of this ,
reported that anti-2b4 antibody stimulation inhibited nk cell activity in immature human nk cells , whereas it enhanced the function of mature cells ( 19 ) .
these differential effects were postulated to result from expression of sap in mature but not in immature nk cells .
this notion is also consistent with the finding made by the same group that 2b4 ligation was inhibitory rather than activating in nk cells from xlp patients ( 10 ) .
that sap rna was present in mouse lymphokine - activated killer cells but not in bm - derived mouse nk cells ( 16 ) .
however , since the impact of 2b4 deficiency was similar in these two nk cell populations doubts remain regarding the consequences of differential sap expression in mouse nk cells .
other variables could explain a differential capacity of 2b4 to mediate positive or negative signaling ( fig . 1 b ) , and these variables may be different in human and mouse nk cells . the dominant inhibitory effect of 2b4 in mouse nk cells may relate to their preferential expression of eat-2 over sap . indeed , unlike sap mrna , eat-2 mrna was detectable both in mouse lymphokine - activated killer cells and in bm - derived mouse nk cells ( 16 ) .
in contrast , as suggested by some of the analyses of xlp - derived nk cells , the apparent activating effect of 2b4 in human nk cells may relate to preferential expression of sap .
it is also possible that 2b4-mediated inhibition relates to the capacity of 2b4 to mediate signals in the absence of sap and eat-2 . in support of this
, transient transfection experiments showed that 2b4 was able to associate with shp-2 in cells lacking sap and eat-2 ( 20 ) .
a careful analysis of sap and eat-2 expression in human and mouse nk cells , in addition to a characterization of 2b4-mediated functions and signals in mice lacking sap and/or eat-2 , will test these ideas .
the reports of wang et al . ( 14 ) and lee et al . ( 16 ) yield several important conclusions .
first , they provide firm evidence of the involvement of slam - related receptors in immune regulation .
second , they indicate that the previously published studies using mab stimulation led to some misconceptions regarding the roles of slam family receptors in vivo . this may relate to the fact that the antibodies were blocking rather than stimulating the functions of slam - related receptors .
it is also plausible that the antibodies were affecting only a subset of the functions of slam - related receptors , thus exposing a biased view of their roles . and third , these manuscripts coupled with other published findings suggest that a given slam - related receptor may have the ability to mediate different , potentially even opposite , signals and functions in different cell populations or activation states
. this ability may relate to differential expression of sap - related adaptors ( fig .
1 ) or to other as yet unappreciated factors . such flexibility would give slam - related receptors an exquisite degree of control during an immune response .
 two papers describing mice deficient in signaling lymphocyte activation molecule and 2b4 represent the first accounts of immune phenotypes in animals lacking members of the slam family of receptors . 
 the findings provide definitive evidence of the importance of slam - related receptors in the regulation of t cell , macrophage , and natural killer cell functions . 
The SLAM Family of Immune Cell Receptors.
Defects in CD4
2B4-deficient Mice Reveal an Inhibitory Role for 2B4 in NK Cells.
Lessons from Mice Lacking SLAM-related Receptors.