The potential of sialylated conjugates as modulators of the innate immune system

Abstract

It has been known since 2004 that neutrophil granulocytes are able to release so-called neutrophil extracellular traps (NETs). NETs consist of decondensed DNA decorated with antimicrobial molecules and cytotoxic histones, which can not only capture pathogens but also render them harmless. This process, initially classified as "beneficial suicide", has been getting more and more attention. To date, the actual mechanism of the release of NETs is not fully understood, but it is already clear that NETs also have a dark side. The exaggerated release of NETs seems to be associated with numerous diseases including infertility. Nowadays, it has been suggested that NETs serve as trap for sperm, contributing to infertility. Interestingly, there are physiological mechanisms known that influence the release of NETs. For example, sialylated glycans on the surface of erythrocytes interact with Siglecs on the surface of neutrophils leading to the inhibition NET formation within the circulation, which might be an important mechanism in vertebrates to prevent an exaggerated activation of the immune system. Intriguingly, our analysis demonstrated that the genes encoding for Siglec-1, CD22, MAG and Siglec-15 are conserved since approximately 400 million years and that, for instance, stress influence the expression pattern of these regulative receptors in fish. These results indicate that the first Siglec-dependent mechanisms were already established in lower vertebrates. Siglec-mediated mechanisms may also take place in the female reproductive tract of mammals. Here highly sialylated mucins are produced and released in extraordinary quantities. We detected that cervical mucins might be important to maintain the balance between pathogen defence and pathophysiological manifestations. Our studies showed that, in line with our hypothesis, cervical mucins ensure fertility by inhibiting the release of NETs as well as the activation of neutrophils via their sialic acid residues. Therefore, we assume that a very high level of stimuli would have to be introduced to initiate NET release under physiological conditions within the cervix and that the inhibition of NETosis might contribute to fertility. Furthermore, alpha 2,8-linked oligosialylated nanoparticles were investigated to mimic the effects of a distinct sialic acid-motif, which is present on cervical mucins. These particles were able to inhibit NET release. However, although the release of NETs could be prevented, neutrophils were activated. The DNA decondensed, neutrophil elastase translocated into the cell nucleus and the nuclear and granular membrane ruptured, but the plasma membrane remained intact. Thus, for a complete inhibition differently attached sialic acid residues (alpha2,3- and/or alpha2,6-linked sialic acid resides) on nanoparticles might be necessary as mucins contain all these different sialic acid-motifs. In sum, this work shows that from lower vertebrates to mammals, Siglec-sialic acid interactions are needed to maintain the balance between pathogen defence and pathophysiological manifestations and that the interaction of sialylated structures with Siglecs can be exploited to develop new clinical applications for the modulation of NET release.