CS1, a SLAM family receptor involved in immune regulation, is a therapeutic target in multiple myeloma  André Veillette, Huaijian Guo  Critical Reviews in Oncology / Hematology  Volume 88, Issue 1, Pages 168-177 (October 2013) DOI: 10.1016/j.critrevonc.2013.04.003 Copyright © 2013 Elsevier Ireland Ltd Terms and Conditions

Fig. 1 Primary structure and features of a model SLAM family receptor. The primary structure of a model SLAM family receptor is shown. It contains an extracellular domain with one variable (V)-type immunoglobulin (Ig)-like domain and one constant (C)2-type Ig-like domain. It also has a single transmembrane domain and a cytoplasmic domain with tyrosine-based motifs that undergo phosphorylation when the receptor is engaged by its ligand. Most SLAM family receptors are self-ligands that interact with ligands through the V-type Ig-like domain. Two types of tyrosine-based motifs exist in the cytoplasmic domain. When phosphorylated, immunoreceptor tyrosine-based switch motifs (ITSMs) enable the recruitment of SAP family adaptors, which typically mediate cell activation. Non-ITSMs presumably bind other effectors, which may mediate cell inhibition. Critical Reviews in Oncology / Hematology 2013 88, 168-177DOI: (10.1016/j.critrevonc.2013.04.003) Copyright © 2013 Elsevier Ireland Ltd Terms and Conditions

Fig. 2 Primary structures and features of SAP family adaptors. The primary structures of SAP and EAT-2 are shown. These adaptors contain an amino-terminal Src homology 2 (SH2) domain and a short carboxyl-terminal tail. Through their SH2 domain, they bind phosphorylated immunoreceptor tyrosine-based switch motifs (ITSMs) in SLAM family receptors. In its SH2 domain, SAP also contains an arginine (R)-based motif (centered on R78) that mediates binding to the Src family protein tyrosine kinase Fyn. This permits the coupling of SLAM family receptors to protein tyrosine phosphorylation signals. In its carboxyl-terminal tail, EAT-2 contains a tyrosine (Y)-based motif (centered on Y127), which upon phosphorylation mediates the recruitment of alternative as yet poorly characterized effectors. Phospholipase C (PLC)-γ is a candidate effector of EAT-2. Critical Reviews in Oncology / Hematology 2013 88, 168-177DOI: (10.1016/j.critrevonc.2013.04.003) Copyright © 2013 Elsevier Ireland Ltd Terms and Conditions

Fig. 3 Primary structures, signaling mechanisms and functions of CS1 and its short isoform, CS1-S. (A) The primary structure of the long isoform of CS1 (CS1-L) is shown. It contains an extracellular domain with one variable (V)-type immunoglobulin (Ig)-like domain and one constant (C)2-type Ig-like domain. It also has a single transmembrane segment and a cytoplasmic domain with two conserved tyrosine-based motifs that undergo phosphorylation when the receptor is engaged by its ligand. Tyrosine (Y) 281 is located in a typical immunoreceptor tyrosine-based switch motifs (ITSMs). When Y281 is phosphorylated, it enables the recruitment of EAT-2, which in turn gets phosphorylated in its carboxyl-terminal tail and mediates cell activation. Y261 is not located in an ITSM. When phosphorylated, it presumably binds other effectors, which may mediate cell inhibition. In cells expressing EAT-2, the function of CS1-L is activating. In cells lacking EAT-2, the function of CS1-L is inhibitory. (B) The primary structure of the short isoform of CS1 (CS1-S) is shown. CS1-S is generated by alternative splicing of the CS1-encoding gene, and lacks both Y261 and Y281. Whether CS1 is functionally inactive or mediating alternative functions is not known. Critical Reviews in Oncology / Hematology 2013 88, 168-177DOI: (10.1016/j.critrevonc.2013.04.003) Copyright © 2013 Elsevier Ireland Ltd Terms and Conditions

Fig. 4 Primary mechanism of action of elotuzumab against multiple myeloma. Accumulating evidence indicates that elotuzumab mediates its anti-multiple myeloma (MM) effect primarily through antibody-dependent cellular cytotoxicity (ADCC). In this mechanism, the Fab portion of elotuzumab binds the CS1 expressed on MM cells, while the Fc portion of elotuzumab binds the activating Fc receptor, CD16, on natural killer (NK) cells. These interactions trigger NK cell activation, release of cytotoxic granules and killing of MM cells. In addition, activated NK cells may release cytokines, such as interferon-γ, that stimulate the recruitment of other immune cell types and augment further the anti-MM effect of elotuzumab (not shown). Critical Reviews in Oncology / Hematology 2013 88, 168-177DOI: (10.1016/j.critrevonc.2013.04.003) Copyright © 2013 Elsevier Ireland Ltd Terms and Conditions

Fig. 5 Possible alternative mechanisms of action of elotuzumab against multiple myeloma. (A) It is possible that elotuzumab also directly activates NK cells by triggering the CS1 expressed on NK cells. CS1 is activating on NK cells, as these cells express the SAP family adaptor EAT-2. This further augments the ability of NK cells to kill MM cells. (B) There is also indication that elotuzumab interferes with the ability of MM cells to adhere to bone marrow stromal cells (BMSCs). Although the mechanism of this effect is not known, it may relate to a direct effect of CS1 engagement on MM cells (model depicted). Alternatively, if BMSCs express CS1, it may relate to a direct impact on BMSCs (model not shown). (C) Elotuzumab may also interfere with the ability of myeloid-derived suppressor cells (MDSCs) to suppress anti-myeloma immune responses by T cells or other immune cells. Since MDSCs are derived from myeloid or monocyte precursors, they may express CS1. Critical Reviews in Oncology / Hematology 2013 88, 168-177DOI: (10.1016/j.critrevonc.2013.04.003) Copyright © 2013 Elsevier Ireland Ltd Terms and Conditions