How the 4% grafting of mannobiose alters the properties of the polyelectrolyte, linear polyethylenimine (PEI), which has protonable amines and is used widely for complexing DNA, is investigated. The pH‐dependence of the PEI charge, zeta potential, and hydrodynamic radius changes when the polymer charge increases beyond 50–60%. According to MD simulations, this transition in pH‐ or charge‐ dependence is a short‐range structural response to amine protonation (i.e., change in intrinsic persistence length) rather than a longer‐range charge‐repulsion response. Upon mannobiosylation, the 50% charge state of PEI shifts from ≈pH 6 toward ≈pH 4. The protonability of PEI is lowered and restricted to limited charge states. These mannobiosylation‐induced shifts are reversed by salt and uncharged chaotropes, indicating that hydrogen‐bonding interactions fostered by the grafted sugars are lowering the protonation pKa of PEI. These findings have broad implications for the analytical modeling of polyelectrolytes, the design of PEI‐based gene‐delivery systems, and for understanding how oligosaccharides modify cell and protein surfaces.

Mannose and sialic acid residues exhibit short-range brittle self-adhesion and long-range tough self-adhesion in both monolayers and complex type N-glycans.

The surfaces of cells and pathogens are covered with short polymers of sugars known as glycans. Complex N-glycans have a core of three mannose sugars with distal repeats of N-acetylglucosamine and galactose sugars terminating with sialic acid (SA). Long-range tough and short-range brittle self-adhesions were observed between SA and mannose residues, respectively, in ill-defined artificial monolayers. We investigated if and how these adhesions translate when the residues are presented in N-glycan architecture with SA at the surface and mannose at the core and with other glycan sugars. Two pseudotyped viruses with complex N-glycan shields were brought together in force spectroscopy (FS). At higher ramp rates, slime-like adhesions were observed between the shields, whereas Velcro-like adhesions were observed at lower rates. The higher approach rates compress the virus as a whole, and the self-adhesion between the surface SA is sampled. At the lower ramp rates, however, the complex glycan shield is penetrated and adhesion from the mannose core is accessed. The slime-like and Velcro-like adhesions were lost when SA and mannose were cleaved, respectively. While virus self-adhesion in forced contact was modulated by glycan penetrability, the self-aggregation of the freely diffusing virus was only determined by the surface sugar. Mannose-terminal viruses self-aggregated in solution, and SA-terminal ones required Ca2+ ions to self-aggregate. Viruses with galactose or N-acetylglucosamine surfaces did not self-aggregate, irrespective of whether or not a mannose core was present below the N-acetylglucosamine surface. Well-defined rules appear to govern the self-adhesion and -aggregation of N-glycosylated surfaces, regardless of whether the sugars are presented in an ill-defined monolayer, or N-glycan, or even polymer architecture.

Receptors that help immune cells recognize 'self' cells can be functionally blocked with antibodies, and a fraction of cancer patients show durable cures through blockade of such T-cell checkpoints. Macrophages possess a similar checkpoint that we block in combination with tumor opsonization in order to target injected macrophages for phagocytosis of cancer cells. The approaches require immune cell infiltration, which motivates study of the physical properties of tumors. We are specifically blocking signal regulatory protein alpha (SIRPa) on macrophages, which binds the ubiquitous 'self'-marker CD47, and we inject these cells together with a pro-phagocytic antibody against tyrosinase-related protein 1 (Tyrp1) on syngeneic B16 melanoma in an immunocompetent mouse. This widely used preclinical murine model for evaluating immunotherapies is sometimes described as a 'solid' tumor, but macropipette aspiration of fresh tumors reveals high compliance and largely irreversible deformation. Such properties will generally modulate macrophage infiltration, interactions, and phenotype. As we continue to study such effects, we already find that systematic injection of engineered macrophages into mice with subcutaneous B16 tumors results in delayed tumor growth and even some durable cures.

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