Assessment of Scavenger Receptor Cysteine-Rich Domain Binding to Bacteria

Abstract

The scavenger receptor cysteine-rich SRCR domain is an ancient protein domain found in SR-A and SR-I scavenger receptors, which is characterized by a conserved arrangement of cysteines (Martinez et al., Pharmacol Rev 63(4):967–1000, 2011; Sarrias et al., Crit Rev Immunol 24(1):1–37, 2004; Telfer and Baldwin, Cell Immunol 296(1):76–86, 2015; PrabhuDas et al., J Immunol, 2017. 198(10):3775–3789). SRCR domains are divided into group A and group B SRCR domains by virtue of how many cysteines they contain and the resulting disulfide bonding pattern. Group B SRCR domains, found in WC1, CD163, CD5, CD6, Spα and DMBT1, are approximately 100–110 amino acids long and contain 6–8 cysteines predicted to form 3–4 disulfide bonds. The crystal structure of a CD5 group B SRCR domain predicts a fold of two beta-sheets and an alpha helix (Rodamilans et al., J Biol Chem 282(17):12669–12677, 2007; Wang et al., Mol Immunol 48:801–809, 2011). SRCR domains bind to many different types of chemical compounds found on cells, viruses, and microbes and are usually found in multiples in the extracellular domains of transmembrane proteins or in secreted proteins. Small amino acid differences between these SRCR domains lead to significant differences in binding affinity. In addition, SRCR domain genes contain allelic polymorphisms and can be extensively duplicated. Thus, single and duplicated SRCR domain protein gene loci encode a large tunable binding potential. Binding to pathogen-associated molecular patterns (PAMPs) combined with signaling potential predicts an important role for these molecules in the immune response. WC1 SRCR domains bind to the spirochetes Leptospira and Borrelia (Hsu et al., J Immunol 194(5):2280–2288, 2015). CD6 (Sarrias et al., Proc Natl Acad Sci U S A 104(28):11724–11729, 2007), Spα (Sarrias et al., J Biol Chem 280(42):35391–35398, 2005), CD163A (Fabriek et al., Blood 113(4):887–892, 2009) and DMBT1 (Madsen et al., Eur J Immunol 33(8):2327–2336, 2003) bind to Gram-positive and Gram-negative bacteria; CD5 binds to yeast (Vera et al., Proc Natl Acad Sci U S A 106(5):1506–1511, 2009). Identified ligands include lipoteichoic acid, lipopolysaccharide, poly-phosphorylated, and -sulfated compounds such as dextran sulfate sodium, leucine-rich repeat proteins, and fungal mannose (Sarrias et al., Proc Natl Acad Sci U S A 104(28):11724–11729, 2007; Sarrias et al., J Biol Chem 280(42):35391–35398, 2005; Fabriek et al., Blood 113(4):887–892, 2009; Vera et al., Proc Natl Acad Sci U S A 106(5):1506–1511, 2009; End et al., Eur J Immunol 39(3):833–842, 2009; Loimaranta et al., J Biol Chem 284(28):18614–18623, 2009). A conserved linear binding motif (VEVLXXXXW) in an external loop in the SRCR domain has been identified in CD163A and DMBT1 and can be used as a peptide that aggregates bacteria (Fabriek et al., Blood 113(4):887–892, 2009; Bikker et al., J Biol Chem 279(46):47699–47703, 2004; Leito et al., Biol Chem 389(9):1193–1200, 2008). In contrast, WC1 binding to bacteria is mediated by a noncontinuous motif in the native protein, and mutation of the VEVLXXXXW motif has no effect upon bacterial binding (Hsu et al., J Immunol 194(5):2280–2288, 2015). Thus, bacterial binding studies with WC1 SRCR domains must be done with native, correctly disulfide bonded, protein, ideally posttranslationally modified in mammalian cells.

WC1 is found in the genomes of most mammals, reptiles, and birds and is expressed exclusively on γδ T cells in ruminants. The 13 bovine WC1 genes encode up to 11 extracellular SRCR domains, organized in the SRCR domain pattern of a1-[b2-c3-d4-e5-d6]-[b7-c8-d9-e10-d’11], where the alphabet designations indicate homology between genes and across species (Chen et al., BMC Genet 13:86, 2012; Herzig et al., BMC Evol Biol 10:181, 2010; Herzig and Baldwin, BMC Genomics 10:191, 2009). Some of the signaling co-receptor WC1 molecules are required for the γδ T cell response to Leptospira (Wang et al., Mol Immunol 48:801–809, 2011; Rogers et al., J Immunol 174(6):3386–3393, 2005; Wang et al., Eur J Immunol 39(1):254–266, 2009). The WC1 expressed on responsive γδ T cells is correlated with its direct binding to Leptospira via some of its SRCR domains (Hsu et al., J Immunol 194(5):2280–2288, 2015). Because WC1⁺ γδ T cells share a restriction in their γδ TCRs and WC1 has TCR co-receptor activity, we hypothesize that WC1 co-ligation with the TCR plays the determining role in the activation of WC1+ γδ T cells by pathogens. Classification of the binding of WC1 SRCR domains, their ligands, and their role in the interaction of 𝛾δ T cells with pathogens relevant to the host will allow these cells to be recruited in next-generation vaccines to pathogens that have significant negative economic and health impact.