TX, USA2B4 (CD244), a member of the signaling lymphocyte-activation molecule (SLAM/CD150), is expressed on all NK cells, a subpopulation of T cells, monocytes and basophils. Human NK cells express two isoforms of 2B4, h2B4-A and h2B4-B that differ in a small portion of the extracellular domain. In the present investigation, we have studied the functions of h2B4-A and h2B4-B. Our study demonstrated that these two isoforms differ in their binding affinity for CD48, which results in differential cytotoxic activity as well as intracellular calcium release by NK cells upon target cell recognition. Analysis of the predicted 3-D structure of the two isoforms showed conformational differences that could account for their differences in binding affinity to CD48. h2B4-A was able to mediate natural cytotoxicity against CD48-expressing K562 target cells and induce intracellular calcium release, whereas h2B4-B showed no effects. NK-92MI, U937, THP-1, KU812, primary monocytes, basophils and NK cells showed expression of both h2B4-A and h2B4-B whereas YT and IL-2-activated NK cells did not show any h2B4-B expression. Stimulation of NK cells through 2B4 resulted in decreased mRNA levels of both h2B4-A and h2B4-B indicating that down-regulation of 2B4 isoforms may be an important factor in controlling NK cell activation during immune responses.Key words: Cell activation . Cell surface molecules . NK cells Introduction NK cells are a fundamental component of the innate immune system, capable of recognizing and destroying tumor cells as well as cells that have been infected by viruses or bacteria [1]. In addition to their role in innate immune responses, interactions between NK cells and dendritic cells regulate the adaptive immune response to pathogens [2]. NK cell functions are regulated by a dynamic interplay between activating and inhibitory signals transmitted by distinct classes of receptors found on their surface [3,4]. The balance between positivesignaling receptors and negative-signaling receptors ultimately determines the outcome of NK cell-target cell encounters [5,6].2B4 (CD244), belongs to the signaling lymphocyte-activation molecule (SLAM) family of receptors, which includes SLAM (CD150), CD84, NTB-A (Ly-108), lymphocyte antigen 9 (Ly9; CD229), and CD2-subset 1 (CS1) (also termed CRACC or CD319) [7][8][9][10]. 2B4 is expressed by all NK cells, as well as a subset of memory CD8 1 abT cells, gdT cells, basophils, monocytes and eosinophils [11][12][13][14][15]. 2B4 binds to the glycophosphoinositol anchored protein, CD48, expressed on all hematopoietic cells, with a similar affinity in mouse and human [16,17]. Engagement of 2B4 via interaction with specific antibodies or CD48 induces cytokine secretion (IFN-g) and enhances non-MHC-restricted 1632killing by NK cells [12,16,18]. 2B4 can also function as a ligand on target cells and stimulate NK cells through the engagement of CD48 [19]. 2B4 was initially identified as an activating receptor in non-MHC-restricted killing of NK and T cells [12,20]. Some studies with human ...

Natural killer (NK) cells are a subpopulation of lymphocytes that represent the first line of defense against stressed, virally infected and malignant cells. NK cell functions are controlled by a balance between positive and negative signals through various receptors. We have identified, cloned and characterized the 2B4 (CD244) receptor in mice and human. 2B4 interaction with its ligand CD48 regulates NK, T and B cell functions and thus plays an important role in various immune responses. Human NK cells express two isoforms of 2B4, h2B4‐A and h2B4‐B that differ in a small portion of the extracellular domain. In the present investigation, we have studied the functions of h2B4‐A and h2B4‐B. Our data demonstrate that these two isoforms differ in their binding affinity for CD48 and this result in differential cytotoxic activity by NK cells against tumor targets. h2B4‐A was able to mediate natural cytotoxicity against CD48 expressing K562 target cells confirming that h2B4‐A is an activating receptor, whereas h2B4‐B showed no effects. Analysis of the predicted 3‐D structure of the two isoforms showed conformational differences which may be responsible for their differences in binding affinity to CD48. Thus differential expression of 2B4 isoforms by NK cells may regulate immune response mediated through 2B4‐CD48 interactions. This work was supported by the NIH grant CA85753.