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
If no other means are available, in rare circumstances it may be necessary for a medical nanorobot to kill a phagocyte that is ingesting it. This is not a desirable procedure unless the total systemic nanorobot dose is extremely light, since it would be easy for even a modest number of active nanorobots to quickly deplete a significant fraction of the body’s professional phagocytes, substantially impairing the reticuloendothelial system and possibly contributing to inflammatory and autoimmune disease . Furthermore, a nanorobot that permanently poisons a phagocyte is a potentially harmful device that may not pass governmental regulatory muster. Many of the techniques of cytocide and virucide summarized in Section 10.4 may be applicable to phagocytes. However, it is important to recall that an anergic or apoptotic outcome is cleaner and thus is preferable to a necrotic outcome – if cell death has become inevitable – for the reasons described in Section 10.4.1.
Before internalization, nanorobots may kill phagocytes mechanically (Section 10.4.2) or chemically (e.g., GdCl3 , beryllium phosphate , or dichloromethylene diphosphonate ). Antigen overstimulation of T cells, a phenomenon known as high-dose suppression, can also induce T-cell death . Various substances produced by pathogens that cause damage to phagocytes have been called aggressins [3302, 3586-3590]. Most aggressins are extracellular enzymes or toxins that kill phagocytes, and include:
(1) streptolysin O (an oxygen-labile thiol-activated cytolysin) from Streptococcus pyogenes (ovoid cocci 0.6-1 micron in diameter ) that binds to cholesterol in the membranes of mammalian cells and organelles  with various cytotoxic effects [3592, 3593];
(2) leukocidal toxins including gamma-hemolysins and leukocidin [3594-3597] from Gram-positive pyogenic cocci such as Staphylococcus aureus [3594-3597] and Staphylococcus intermedius , and cell-bound leukocidin from Pseudomonas aeruginosa ;
(3) exotoxin A, a bacterial extracellular protein of Pseudomonas aeruginosa that inhibits protein synthesis by ~50% and kills macrophages in ~1 hour at ~10 ng/ml [3599-3601]; and
(4) various substances secreted by Enterococci with cytolytic toxicity for phagocytic cells [3301, 3602].
After internalization, nanorobots may kill phagocytes chemically, for example, by toxifying the cell via intracellular acidosis [3603-3608] from CO2 gas release from onboard pressure tanks  with sufficient speed and quantity to overcome the bicarbonate buffer system. Like lymphocytes, erythrocytes, and platelets  (but unlike brain [3606-3608] and muscle  cells), alveolar macrophages are permeable  to H+/HCO3-. Measured mean cytosolic pH is 7.1 in rat renal epithelial cells , 7.09-7.19 in rat cardiac myocytes , 7.18-7.21 in rat fibroblasts , 7.21 in normal human platelets , 7.33 in human erythrocytes , and 7.39 in human lymphocytes . Lethal intracellular CO2 acidosis in nonpermeable cells is approximately 5.8-6.2 , so CO2-induced lethality requires a decrease in intracellular pH of at most 1.6, corresponding to the injection of 5.2 million CO2 molecules (~0.1% of respirocyte  storage capacity) into a (20 micron)3 cell comprised 70% of water. Of course, artificial CO2 intracellular acidosis elicits a restorative alkalinization response  including extracellular transport , thus likely necessitating a somewhat higher intracellular lethal dose to be administered by nanorobots, in actual practice.
Nanorobots also may induce phagocyte death much like the intracellular parasites of macrophages such as Mycobacterium, Brucella, and Listeria – for instance, via lymphokine-activated killer-mediated cytolysis of monocytes chronically infected with mycobacteria . A more direct example is offered by the malarial (Plasmodium) sporozoites [3616-3618]. These enter the fixed phagocytes of the liver (Kupffer cells) enclosed in a vacuole that resists phagolysosomal fusion (Section 220.127.116.11.7). But before forming a parasitophorous vacuole, the sporozoites can travel completely through the body of the fixed phagocyte and exit the Kupffer cell on the other side . Then they invade the hepatocytes (other liver cells) that lie adjacent to the Kupffer cell. The sporozoites accomplish this by releasing into the phagocyte cytosol a considerable amount of circumsporozoite (CS) protein, a ribotoxic agent that inhibits phagocytic protein synthesis and selectively kills the Kupffer cells through which the sporozoites pass .
It is believed that all eukaryotic cells, including phagocytes, incorporate an evolutionarily conserved self-destruct mechanism called programmed cell death or apoptosis (Section 10.4.1.1). This is an intracellular cascade of genetically predetermined biochemical steps in which the cell disassembles itself in an orderly manner, in 30-60 minutes . Phagocyte apoptosis may be triggered by various means. For example, B lymphocytes and T lymphocytes undergo apoptosis in response to anti-IgM antibodies and dexamethasone (a glucocorticoid), respectively . Exposure to 4.25% solution of glucose-lactate-based peritoneal dialysates elicits accelerated apoptosis in cultured phagocytes (monocytes and neutrophils) . Phorbol 12-myristate 13-acetate (PMA) induces morphological degeneration and cell death in 3-6 hours in porcine PMNs in vitro . Apoptosis-inducing CD95L ligand expression has already been mentioned in connection with immune privilege (Section 18.104.22.168). Shigella flexneri produces apoptosis in cultured macrophages via IpaB protein secretion – IpaB binds to interleukin-beta-converting enzyme (ICE), a cysteine protease that can initiate apoptosis when expressed in cells . In vivo, Shigella flexneri induces extensive apoptosis of macrophages, B cells, and T cells located under M cells in the intestinal walls . Yop proteins from Yersinia species signal macrophages to undergo rapid apoptosis [3624-3627]. Salmonella typhimurium also induces apoptosis in macrophages [3389, 3628], as does Listeria during listeriosis in infected hepatocytes , both in vivo and in vitro, mediated by listeriolysin O .
It should be possible to design explicit biological-derived autodestruct mechanisms into biorobots (Chapter 19) that are analogous to apoptosis. However, disposability engineering for diamondoid nanorobots which allows for biocompatible planned biodegradability – nanorobot apoptosis – will be difficult to accomplish (Section 22.214.171.124) and would probably present higher risks due to reduced process control during intermediate stages of self-disassembly.
Last updated on 30 April 2004