Parasitic nematodes manufacture various carbohydrate-linked phosphorylcholine (PCh)-containing molecules, including ES-62, a protein with an N-linked glycan terminally substituted with PCh. The PCh component is biologically important because it is required for immunomodulatory effects. We showed that most ES-62 was bound to a single protein, C-reactive protein (CRP), in normal human serum, displaying a calcium-dependent, high-avidity interaction and ability to form large complexes. Unexpectedly, CRP binding to ES-62 failed to efficiently activate complement as far as the C3 convertase stage in comparison with PCh-BSA and PCh-containing Streptococcus pneumoniae cell wall polysaccharide. C1q capture assays demonstrated an ES-62-CRP-C1q interaction in serum. The three ligands all activated C1 and generated C4b to similar extents. However, a C2a active site was not generated following ES-62 binding to CRP, demonstrating that C2 cleavage was far less efficient for ES-62-containing complexes. We proposed that failure of C2 cleavage was due to the flexible nature of carbohydrate-bound PCh and that reduced proximity of the C1 complex was the reason that C2 was poorly cleaved. This was confirmed using synthetic analogues that were similar to ES-62 only in respect of having a flexible PCh. Furthermore, ES-62 was shown to deplete early complement components, such as the rate-limiting C4, following CRP interaction and thereby inhibit classical pathway activation. Thus, flexible PCh-glycan represents a novel mechanism for subversion of complement activation. These data illustrate the importance of the rate-limiting C4/C2 stage of complement activation and reveal a new addition to the repertoire of ES-62 immunomodulatory mechanisms with possible therapeutic applications.

Aims To improve the tolerability and therapeutic application of histone deacetylase inhibitors (HDACi), by application of an esterase‐sensitive motif (ESM), to target pharmacological activity directly to mononuclear myeloid cells expressing the processing enzyme carboxylesterase‐1 (CES1). Methods This first‐in‐human study comprised single and multiple ascending dose cohorts to determine safety and tolerability. Pharmacodynamic parameters included acetylation, cytokine inhibition and intracellular concentrations of processed acid metabolite in isolated monocytes. Mechanistic work was conducted in vitro and in a CES1/Es1elo mouse strain. Results ESM‐HDAC391 showed transient systemic exposure (plasma half‐life of 21–30 min) but selective retention of processed acid for at least 12 hours, resulting in robust targeted mechanistic engagement (increased acetylation in monocytes plus inhibition of ex vivo stimulated cytokine production). ESM‐HDAC391 was well tolerated and clinical toxicities common to non‐targeted HDACi were not observed. ESM‐HDAC391 treatment was accompanied by the novel finding of a dose‐dependent monocyte depletion that was transient and reversible and which plateaued at 0.06 × 109 monocytes/L after repeat dosing with 20 or 40 mg. Characterisation of monocyte depletion in transgenic mice (CES1/Es1elo) suggested that colony stimulating factor 1 receptor loss on circulating cells contributed to ESM‐HDAC‐mediated depletion. Further mechanistic investigations using human monocytes in vitro demonstrated HDACi‐mediated change in myeloid fate through modulation of colony stimulating factor 1 receptor and downstream effects on cell differentiation. Conclusion These findings demonstrate selective targeting of monocytes in humans using the ESM approach and identify monocytopaenia as a novel outcome of ESM‐HDACi treatment, with implications for potential benefit of these molecules in myeloid‐driven diseases.