© 2003 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume IIA: Biocompatibility, Landes Bioscience, Georgetown, TX, 2003

15.4.3.2.4 Phagocytosis in Spleen Vasculature

The spleen is an “immunological conference center” [2941] that may be thought of as two distinct organs [361] – (1) an immune organ (the white pulp [2942]) consisting of periarterial lymphatic sheaths and germinal centers comprised mainly of 40- to 200-micron³ splenic lymphocytes [2943, 2944]; and (2) a phagocytic organ (the red pulp) consisting of granulocytes (e.g., neutrophils and eosinophils), NK cells (~25% of splenic lymphocytes [2945]), and macrophages either lining the vascular spaces (i.e., the splenic cords and sinusoids; Section 15.4.2.3) or resident in the perifollicular and marginal zones [2946].

Splenic macrophages resemble Kupffer cells (Section 15.4.3.2.3) in morphology and functional properties [2947]. These macrophages remove from passing blood, via phagocytosis, certain parasites [2948], bacteria [2949], worn out blood cells (red cells [2950-2952], white cells [2950], platelets [2950, 2953]), and other particles [2950]. Splenic macrophages also break down red cell hemoglobin into the pigment bilirubin [2954, 2955], which is released into the blood plasma and subsequently removed by liver, marrow and kidneys. (The spleen clears mildly damaged erythrocytes from the circulation, whereas more severely damaged red cells are removed mainly by the liver [2863].) Approximately 10¹¹ erythrocytes/day are phagocytized by macrophages in the red pulp cords [2956]. Mean turnover time for murine splenic macrophages is 6 days, with 55% of the macrophage population supplied by monocyte influx and 45% by local production [2957].

There have been relatively few direct investigations of the propensity of splenic macrophages to ingest inert particles that might be analogous to medical nanorobots, though the results should be similar to Kupffer cells. One line of experiments found that uncoated 0.1-micron polystyrene microspheres experience only ~1% uptake by rat spleen after 24 hours in circulation, whereas 0.22-micron particles have ~5% splenic uptake and 0.5-micron microspheres experience ~30% uptake within 24 hours of IV administration [2904]. If the same microspheres are coated with poloxamine-908 surfactant, and if particle injection is preceded by 1-3 hours with a predosing of free poloxamine-908, then splenic uptake of these larger microspheres is dramatically reduced, but this is due to increased accumulation in hepatic Kupffer cells [2958, 2959] and not to altered affinity of splenic macrophages for microspheres. Indeed, without the predosing, splenic uptake is dramatically increased [2735].

Another experiment found that challenging mouse RES with colloidal carbon produced only an increase in the population of splenic lymphocytes, although the thymus underwent acute cortical atrophy followed by post-challenge cellular replenishment [2960]. Phagocytosis of colloidal carbon by splenic macrophages takes place within 20-30 seconds of IV injection, mostly by macrophages from the Billroth’s cords and not by sinus-lining endothelial cells. After 24 hours, the particles are still mostly in the red pulp, with a small number in the periphery of the white pulp but never diffusely throughout this area [2961]. Percoll microspheres 20-30 nm in diameter can also reach the thymic cortex from the murine intestinal lumen, there to be absorbed by perivascular thymic macrophages [2962]. Hydrophilized nanospheres <0.1 microns in diameter show negligible uptake by splenic or hepatic macrophages; increasing particle size or hydrophobicity increases RES uptake [2963].

Assuming that completely passive nanorobots are ingested by splenic phagocytes, what might be the fate of these particles? Once again, experimental studies are few. Macrophages heavily laden with inert carbon particles, when injected into rat splenic artery, were found to slowly migrate from the red pulp marginal zone to the periphery of the white pulp, into the deeper white pulp, and finally into the germinal centers [2964]. Limited numbers of macrophages made the journey in 12-24 hours, but most had completed their journey into the lymphatic tissue after 10 days [2965]. Latex microspheres do not induce granuloma formation in murine spleen cells in vitro, but dextran microparticles do [2966]. Granulomatogenesis apparently can be suppressed by the addition of dexamethasone, PGE2, or certain T cell-derived lymphokines such as IL-4 and IFN-gamma [2967]. Massive overdoses of 0.05-micron magnetite-dextran nanoparticles have produced splenomegaly in mice [2968], and IV injections of metallic tin powder particles in rats have produced up to six-fold splenomegaly and epithelioid granulomas [2969]. Clearly an active phagocyte escape protocol (Section 15.4.3.6) would provide a useful capability for medical nanorobots in transit through the spleen. Particle clearance from the lymphatics is briefly discussed in Section 15.4.3.4.

Last updated on 30 April 2004