Nanomedicine, Volume IIA: Biocompatibility
© 2003 Robert A. Freitas Jr. All Rights Reserved.
Robert A. Freitas Jr., Nanomedicine, Volume IIA: Biocompatibility, Landes Bioscience, Georgetown, TX, 2003
126.96.36.199.2 Avoid Phagocytic Recognition
Chemorepulsion is adequate for a few devices on simple missions of limited duration (Section 188.8.131.52.1), but large numbers of medical nanorobots on longer or more complex missions will inevitably come into physical contact with many phagocytes. The least disruption to normal immune processes is achieved if the nanorobot surface can deny recognition to the inquiring phagocyte at the moment of physical contact. Surface-bound moieties are generally preferable to free-released molecules when large populations of in vivo nanorobots are involved. For example, each nanorobotic member of an internal communication network (Section 7.3.2), stationed perhaps ~100 microns apart throughout the tissues, must continuously avoid being ingested by passing phagocytes. Any approach that relies primarily on antiphagocytic chemical releases risks extinguishing all phagocytic activity throughout the body, potentially compromising the natural immune system.
In 2002, “long-circulating” phagocytosis-resistant particles [1450, 2487, 2488, 2491, 5051-5057], stealth drug carriers [1480, 1481, 2682] and stealth nanoparticles [3325, 3326, 5058, 5059] were the objects of active research. It was well known that nanoparticle adsorption and internalization by phagocytes could be inhibited by the presence of a coating of polysaccharide (e.g., heparin or dextran) chains in a brush-like configuration [2490, 3325], or by very hydrophilic coatings (Section 184.108.40.206). Low phagocytic uptake can be achieved using a surface concentration of 2-5% by weight of PEG. This gives efficient steric stabilization (e.g., a distance of ~1.5 nm between two adjacent terminally-attached PEG chains in the covering brush ) and avoids uptake by PMN cells . Experiments by Illum, Davis, et al [2682, 3003] suggest that polystyrene particles sterically stabilized with adsorbed poloxamer polymer could achieve an extrapolated zero phagocytic uptake using a ~10 nm thick coating on 60 nm diameter particles or a ~23 nm thick coating for 5.25-micron diameter particles, thus eliminating nonspecific phagocytosis. Another study  found that pegylated sheep RBCs were ineffectively phagocytosed by human monocytes, unlike untreated sheep RBCs. Phagocytosis of polystyrene beads (as measured by cellular oxygen consumption) appears strongly dependent on surface potential and thus upon fixed surface charge , and surface charge heterogeneity across domains as small as 1-4 microns can greatly affect phagocytic ability .
Rather than coatings which phagocytes cannot recognize at all, medical nanorobots alternatively could carry surfaces that phagocytes will recognize as “self.” For example, coatings that mimic natural immune-privileged cells (Section 220.127.116.11) could be used. Nanorobot exteriors could be covalently bound with essential erythrocyte coat components – a simulated RBC surface could be useful in the bloodstream, though it might provoke a response in the tissues. Similarly, immune-blind fibroblast-like surface might be useful especially in the tissues, and even in blood – while bloodborne fibroblast lysate does elicit a response , some fibroblasts may originate from peripheral blood cells  called fibrocytes  which have been observed to differentiate into fibroblasts , and bone marrow-derived fibroblast CFUs are also observed in blood . A metamorphic surface that alternated between these two displays, depending upon location in the body, might be feasible (Section 5.3.6). But a simulated neutrophil or monocyte surface would be better, since these cells normally migrate from blood to tissues, hence the immune system expects to see these surfaces virtually everywhere. Lymphocytes are likewise normally present in both blood and tissues, and are also adept at passing through the endothelial lining, the lymphatic processes, and the lymph nodes without being detained or trapped, eventually returning to the arterial circulation . The ideal solution may be for the medical nanorobot to display a specific designed set of self-markers at its surface. These markers might include moieties such as CD47, aka. integrin associated protein or IAP. CD47 is a surface protein present on almost every cell type that provides an explicit phagocytic inhibitor signal to NK cells and to macrophages .
Microbial pathogens have employed a similar strategy to create antiphagocytic surfaces that avoid provoking an overwhelming inflammatory response, thus preventing the host from focusing the phagocytic defenses . Enveloped viruses and some bacterial pathogens can cover their external cell surface with components that are seen as “self” by the host’s phagocytes and immune system, a strategy that hides the true antigenic surface (Section 18.104.22.168). Phagocytes then cannot recognize the bacterium upon contact and the possibility of opsonization by antibodies to enhance phagocytosis is minimized .
For example, Group A streptococci can synthesize a capsule composed of hyaluronic acid, the “ground substance” (tissue cement) found in human connective tissue [2335, 3302, 3307-3309]. The streptococcal hyaluronic acid capsule is nonantigenic and thus very effective in preventing attachment of the organism to the macrophage . Additionally, the cytoplasmic membrane of Streptococcus pyogenes contains antigens similar to those found on human cardiac , skeletal  and smooth muscle cells, on heart valve fibroblasts, and in neuronal tissues, resulting in molecular mimicry and an immune tolerance response by the host [3309-3311]. Pathogenic Staphylococcus aureus produces cell-bound coagulase which clots fibrin on the bacterial surface [1723-1727, 3302]; the syphilitic agent Treponema pallidum binds human fibronectin to its surface [1731, 3302]; and a variety of bacteria cause meningitis while avoiding phagocytosis either (1) by preventing deposition of complement by sialic acid on the surface, or (2) by modification of lipopolysaccharide (LPS)  (to which the immune system is unusually sensitive). Another example of antiphagocytic surfaces is presented by Haemophilus influenza, which expresses a mucoid polysaccharide capsule that prevents digestion by host phagocytes . A few strains resist opsonization and have become serum resistant by modifying their LPS O-antigen side chains, rendering them “invisible” to host immune defenses .
Bacteriophages, viruses first employed against bacteria by d’Herelle in 1922 , are self-replicating pharmaceutical agents  that can grow inside of and destroy pathogenic bacteria when injected into infected hosts during “phage therapy” [3331-3344, 5758-5763, 6211]. Phage biocompatibility is being investigated . Even in the absence of an immune response, intravenous therapeutic phage particles are rapidly eliminated from circulation by the RES, largely by sequestration in the spleen . But Merril et al  found that splenic capture could be greatly eliminated by the serial passage of phage through the circulations of mice to isolate mutants that resist sequestration. This selection process resulted in the modification of the nature of the phage surface proteins, via a double-charge change from acidic to basic, which is achieved by replacing glutamic acid (- charge) with lysine (+ charge) at the solvent-exposed surface of the phage virion. The mutant virions display 13,000-fold to 16,000-fold greater capacity to evade RES entrapment 24 hours post-injection as compared to the original phage . Similar surface modifications can be designed for use on medical nanorobots.
Last updated on 30 April 2004