CD74 (Invariant chain; Ii) is a type II integral membrane protein that is expressed on antigen-presenting cells, and was initially thought to function mainly as a major histocompatibility complex (MHC) class II chaperone (Stumptner-Cuvelette and Benaroch, 2002). A small proportion of CD74 is modified by the addition of chondroitin sulfate, and this form of CD74 is expressed on the cell surface. The cell surface CD74 molecule serves as a receptor in many types of cells and can initiate various signalling cascades (Naujokas et al., 1993; Maharshak et al., 2010). The cytokine, MIF, was found to be the natural ligand of CD74. MIF binds to the extracellular domain of CD74 with high affinity (KD = 1.40 × 10−9 M) and initiates a signalling cascade (Leng et al., 2003). CD74 forms a complex with CD44, which is essential for the MIF-induced signalling cascade (Shi et al., 2006; Gore et al., 2008).
In murine B cells, CD74 expression is directly involved in shaping the B cell repertoire by regulating mature B cell survival (Shachar and Flavell, 1996; Matza et al., 2002; Matza et al., 2003). MIF binding to CD74 induces a signalling pathway that involves the Syk tyrosine kinase and the PI3K/Akt pathway, induction of CD74 intramembrane cleavage and the release of the CD74 intracellular domain (CD74-ICD). CD74-ICD translocates to the nucleus where it induces activation of transcription mediated by the NF-κB p65/RelA homodimer and its co-activator, TAFII105, resulting in the regulation of transcription of genes that control B cell proliferation and survival (Matza et al., 2001; Starlets et al., 2006; Gore et al., 2008). MIF was found to regulate cell entry into the S-phase in a CD74- and CD44–dependent fashion, by elevating cyclin E levels, resulting in cell proliferation. In addition, this cascade augments Bcl-2 expression, further supporting cell survival (Starlets et al., 2006; Gore et al., 2008). Thus, the MIF binding to CD74/CD44 complex initiates a survival pathway, resulting in proliferation of the mature B cell population and prevention of their death.
Activation of the CD74/CD44 complex on B cells augments the cell surface expression of the tyrosine kinase receptor hepatocyte growth factor receptor (HGFR) (c-Met) and secretion of its ligand HGF (Gordin et al., 2010). Following MIF stimulation, HGF engages with CD74 and CD44 on the cell membrane and, together with HGF, triggers a signalling cascade, which is necessary to initiate the MIF-induced survival cascade (Gordin et al., 2010).
MK as a regulator of mature B cell survival
The MIF/CD74/HGFR-induced cascade was recently shown to control expression of MK, a heparin-binding cytokine, in vitro and in vivo (Cohen et al., 2012). Stimulation of cultured splenic B cells with MIF up-regulates MK mRNA and protein levels. Moreover, following MIF injection to C57BL/6 mice, a significant elevation in MK protein was detected, demonstrating the modulation of MK expression by MIF. This activation is CD74-specific, since no change in MK levels was observed following stimulation of CD74 deficient B cells with MIF. Thus, MIF binding to its receptor, CD74, regulates MK expression in B cells (Cohen et al., 2012).
MIF and CD74 regulate the expression of the tyrosine kinase receptor, HGFR, and its ligand, HGF, which are essential for the CD74-induced survival cascade (Gordin et al., 2010). Interestingly, stimulation of HGFR with HGF elevates MK mRNA and protein levels in both wild-type and CD74-deficient B cells. Moreover, blocking c-Met activity using the c-Met inhibitor PHA-665752, a selective small molecule, active-site inhibitor of the catalytic activity of HGFR kinase (Ki 4 nM), which competes with its ATP binding (Ma et al., 2003), or with anti-HGF blocking antibody (Gordin et al., 2010), decreases intracellular protein levels of MK. Together, these results suggest that stimulation of CD74 with MIF leads to an up-regulation in the expression of HGF and HGFR. The binding of HGF to HGFR in turn promotes the expression of MK in mouse splenic B cells ultimately leading to cell survival (Cohen et al., 2012).
MK was previously shown to act as an anti-apoptotic factor in the human haepatoma cell line, HepG2, by down-regulating the activity of caspase-3 (Ohuchida et al., 2004). MK stimulation of cultured splenic B cells triggers the Syk and Akt signalling cascade (Cohen et al., 2012), which elevates the expression of the anti-apoptotic gene, Bcl-2, and inhibits the activity of caspases 3 and 7, leading to mature B cell survival and an elevation in the percentage and number of mature B cells. The MK-induced survival cascade can partially bypass the lack of survival signals transmitted in CD74-deficient B cells. MK is also able to induce survival in cells in which HGFR activity is perturbed, indicating that MK activation is a downstream event to the MIF/CD74 and HGF/HGFR induced survival cascade (Cohen et al., 2012).
Several cell-surface receptors were found to recognize MK, including members of the syndecan family, namely syndecan-1, -3 and -4 (Nakanishi et al., 1997), protein tyrosine phosphatase ζ (PTPRZ1 also known as RPTPζ) (Maeda et al., 1999), transmembrane protein low-density lipoprotein receptor-related protein (Muramatsu et al., 2000), the anaplastic lymphoma kinase (ALK) (Stoica et al., 2002), and the integrins α4β1 and α6β1 (Muramatsu et al., 2004). PTPRZ1 is expressed in normal B cells and was shown to be the most important MK receptor for regulating B cell survival (Cohen et al., 2012). Characterization of the peripheral B cell repertoire of PTPRZ1-deficient mice revealed a significant decrease in the proportion of mature B cells. These results demonstrate the essential role of PTPRZ1 in controlling and shaping the B cell repertoire. Moreover, in the absence of PTPRZ1, neither both MK, MIF nor HGF were able to enhance cell survival, demonstrating that PTPRZ1 is essential for the survival cascade induced by MIF/CD74 and HGF/HGFR (Cohen et al., 2012) (Figure 2).