Evolution of CD33-related siglecs: regulating host immune functions and escaping pathogen exploitation?

Abstract

Sialic-acid-binding immunoglobulin-like lectins, siglecs, are important immune receptors expressed widely in mammals. A unique feature of siglecs is their ability to bind sialylated glycans and transmit signals to immune cells. The CD33-related siglecs (CD33rSiglecs) form a major subfamily of the siglecs, containing a large, rapidly evolving group of genes that expanded in mammals through an inverse duplication event involving a primordial cluster of siglec genes over 180 million years ago. Humans express a much larger set of CD33rSiglecs than mice and rats, a feature that can be explained by a dramatic loss of CD33rSiglec genes in rodents. Most CD33rSiglecs have immune receptor tyrosine-based inhibitory motifs and signal negatively. Interestingly, novel DAP-12-coupled 'activating' CD33rSiglecs have been identified, such as siglec-14 and siglec-16, which are paired with the inhibitory receptors, siglec-5 and siglec-11, respectively. The evolution of these activating receptors may have been driven in part by pathogen exploitation of inhibitory siglecs, thereby providing the host with additional pathways by which to combat these pathogens. Inhibitory siglecs seem to play important and varied roles in the regulation of host immune responses. For example, several CD33rSiglecs have been implicated in the negative regulation of Toll-like receptor signalling during innate responses; siglec-G functions as a negative regulator of B1-cell expansion and appears to suppress inflammatory responses to host-derived 'danger-associated molecular patterns'. Recent work has also shown that engagement of neutrophil-expressed siglec-9 by certain strains of sialylated Group B streptococci can suppress killing responses, thereby providing experimental support for pathogen exploitation of host CD33rSiglecs.

Figure 1

Domain structure of CD33-related sialic-acid-binding immunoglobulin-like lectins (CD33rSiglecs) in human, mouse and dog. Shading represents paired receptors in which the terminal immunoglobulin domains are > 99% identical but the transmembrane and cytoplasmic tails are distinct. + represents a positively charged residue in the transmembrane domain. Dark purple domains are V-set immunoglobulin superfamily (IgSF) domains that mediate sialic acid binding, light purple domains are C2-set IgSF domains. Siglec-8b is the result of a gene duplication of siglec-8 found in dog but not primates or rodents. Atypical immunoreceptor tyrosine-based inhibitory motifs (ITIMs) in dog CD33 and siglec-5 do not conform to canonical ITIM sequence nor do they share similarity to ITIM-like motifs with other siglecs.

Figure 2

Simplified inversed duplication model of CD33-related sialic-acid-binding immunoglobulin-like lectin (CD33rSiglec) evolution. A primordial cluster containing five siglec genes arranged in tandem probably existed before mammalian evolution 180 million years ago (siglec genes in black). This cluster is thought to have also contained non-siglec genes (white) located between the siglec genes. This primordial cluster is thought to have undergone an inverse duplication, creating a larger cluster with two sub-clusters, each containing five siglec genes and two non-siglec genes encoded in opposing directions. This is the initial cluster from which CD33rSiglec clusters of different mammalian species evolved. Species differences are most pronounced when comparing rodents and primates. Rodents show a large-scale deletion of genes from the cluster, resulting in four siglecs only. In contrast, primates maintained most siglecs from the initial cluster and also expanded siglec genes giving rise to both activating (red) and inhibitory (blue) forms. However, most of the newly formed siglecs have become pseudogenes, consistent with a strong de-selective drive.

Figure 3

Evolution of sialic-acid-binding immunoglobulin-like lectin 16 (siglec-16) from siglec-11. An ancestral siglec-11 gene existed before rodents and primates. This gene underwent an inverse duplication in early primates creating two inhibitory siglec-11-like genes. The gene located at the original location of siglec-11 underwent inverse duplication in the chimpanzee to create a new siglec-16 gene containing an activating transmembrane domain and a cytoplasmic tail lacking immunoreceptor tyrosine-based inhibitory motifs (ITIMs). Rodents show a complete deletion of the siglec-11 gene and no inverse duplication (a). Currently siglec-16 wild-type and 4-bp deletion alleles exist at 50%: 50% distribution in the population. We make two hypotheses as to how this ratio came about. In the first hypothesis (upper), after creation of siglec-16, a balance of positive selective pressure from pathogen manipulation of siglec-11 and negative selective pressure from siglec-16 causing an inappropriate immune response has kept the wild-type and 4-bp deletion alleles at continuous flux. In the second hypothesis (lower), after creation of siglec-16, negative selection as the result of inappropriate activation of wild-type siglec-16 has overcome the positive selective effect of the pathogen causing a gradual fading of the wild-type allele and accumulation of the 4-bp deletion mutant in the population. It could also be the case that the pathogen(s) responsible for selection of siglec-16 was simply eliminated from the population after the appearance of siglec-16 (b).

Figure 4

Model for the involvement of sialic-acid-binding immunoglobulin-like lectin G (siglec-G) in danger signalling. It is proposed that CD24 is complexed with siglec-G but does not exhibit inhibitory signalling in the absence of a danger signal such as HMGB1. When there is only pathogen-derived signal, the CD24–siglec-G complex is not engaged and the Toll-like receptor (TLR) is able to deliver activating signals to the immune cell without inhibitory signals from siglec-G. When there is a danger signal, such as HMGB1, both the CD24–siglec-G complex and TLR are engaged. Siglec-G becomes active and is able to suppress signalling from TLR and dampen pro-inflammatory responses.