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

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

15.2.3.6 Immune Evasion

Certain parasites also display a form of immune privilege that is more properly termed “immune evasion” [492-497, 2348], which might also be borrowed for medical nanorobot design. For example, live adult blood fluke (schistosomiasis) worms produce no lesions and rarely cause symptoms [498] or allergic reactions [495]. Schistosome parasites, despite being multicellular organisms up to several millimeters in length, can survive in the bloodstream of mammalian hosts for decades [496] even in the face of an ongoing antiparasite immune response by the infected host [492]. The developmental and adult stages of the parasite are mostly invisible to the immune system [496]. In vitro, bound complement is localized to infoldings of the parasite’s tegument and not on its free surfaces [499]. Adult worms possess surface molecules bearing alternative pathway complement activation sites (Section 15.2.3.2), but these sites are masked by adsorbed host components in vivo [499]. Adsorbed host serum components can also inhibit specific antigen-antibody interactions at the parasite’s surface, suggesting a degree of specificity in what the parasite adsorbs from the host [500] – adult worms can adsorb heterospecific [501] and homospecific [502] antibody onto their tegumental surfaces. Antibody bound to worm tegumental antigen causes shedding of the bound complex in ~20 minutes at 37 ^oC [503].* Soluble adult worm antigen preparation (SWAP) triggers release of cytokine IL-10 from peripheral blood mononuclear cells from both healthy and infected individuals [505], and the IL-10 then suppresses lymphoproliferative responses to SWAP by 90-100% [504]. T cell proliferative hyporesponsiveness [505, 506], nonspecific T-cell immunodepression [507] and modulation of immune responses [508] are well known in chronic schistosomiasis.

* Some bacteria also shed bound antigen-antibody immune complexes [1737].

The human body does not recognize the adult worms as foreign material because, although purified schistosomal tegumental protein is potently immunogenic [509], the adult parasites can remake their surfaces constantly and cover them with native molecules taken from the human host [510]. This covering may include material borrowed from host red cells [511], neutrophils [512], LDLs [513], and other sources [503]. Surface turnover is mostly slow. Immunoradiometric assays show that host erythrocyte antigen is lost from adult worm tegument with a half-life of up to 45 hours in vitro and ~5 days in vivo [493]. The component of adult surface cell lipid bilayer with the fastest turnover is phosphatidylcholine and is due to deacylation/reacylation, not to the sloughing of membranes [497]. Thus a relatively stable adult schistosome surface membrane escapes immune recognition and damage by employing active processes which result in reduced surface antigenicity [495] and the development of a tegument intrinsically resistant to immune damage [492] – a potentially useful example for medical nanorobotics. C. Haberzettl suggests that early simple therapeutic nanorobots might incorporate an “onion-skin” design, with separate concentric layers serving distinct purposes (e.g., organ targeting, cytopenetration, intracellular transport, etc.) and being sloughed off or absorbed in sequence, as their specific purpose is completed.

With an appropriate design, nanorobots could alter their antigenic signature (Section 5.3.6) fast enough to avoid antibodies from being raised at all. Some microbes already employ a related strategy. K. Todar (from whose discussion [1437] the next seven paragraphs draw heavily) points out that a similar example of immune evasion is displayed by Borrelia recurrentis, a spirochete that causes the human disease relapsing fever [1746, 1747]. Explains Todar [1437]: “The disease is characterized by episodes of fever which relapse (come and go) for a period of weeks or months. After infection, the bacteria multiply in tissues and cause a febrile illness until the onset of an immune response a week or so later. Bacteria then disappear from the blood because of antibody mediated phagocytosis, lysis, and agglutination, and then the fever falls. Then an antigenically distinct mutant arises in the infected individual, multiplies, and in 4-10 days reappears in the blood and there is another febrile attack. The immune system is stimulated and responds by conquering the new antigenic variant, but the cycle continues. There may be up to 10 febrile episodes before final recovery. With each attack, a new antigenic variant of the bacterium appears and a new set of antibodies is formed in the host.” This bacterium can change its antigenic signature during the course of an infection in a single host. [1437, 2544] Antigenic variation [1736] usually results from site-specific inversions or gene conversions or gene rearrangements in the DNA of the microbes. Antigenic variation is also found in Plasmodium [1740, 1741], in trypanosomes that can switch between the transcription of one of an estimated 1000 variant surface glycoprotein genes [1750], and in other parasites [1742].

Many pathogenic bacteria exist in nature as multiple antigenic types or serotypes, meaning that they are variant strains of the same pathogenic species [1437]. For example, there are over 1800 known serotypes of Salmonella typhimurium based on differences in cell wall (O) antigens or flagellar (H) antigens [1735]. There are more than 80 different antigenic types of Streptococcus pyogenes based on M-proteins on the cell surface [1734], and over 100 strains of Streptococcus pneumoniae depending on their capsular polysaccharide antigens. Based on minor differences in surface structure chemistry there are multiple serotypes of Vibrio cholerae, Staphylococcus aureus, Escherichia coli, Neisseria gonorrhoeae, and an assortment of other bacterial pathogens. Antigenic variation is prevalent among pathogenic viruses as well.

Neisseria gonorrhoeae can change fimbral antigens during the course of an infection [1437]. During initial stages of an infection, adherence to epithelial cells of the cervix or urethra is mediated by pili (fibriae). Equally efficient attachment to phagocytes would be undesirable. Rapid switching on and off of the genes controlling pili is therefore necessary at different stages of the infection, and N. gonorrhoeae is capable of undergoing this type of “pili switching” or phase variation [1732, 1733]. Genetically controlled changes in outer membrane proteins also occur in the course of an infection. This finely controlled expression of the genes for pili and surface proteins changes the adherence pattern to different host cells, increases resistance to cervical proteases, increases resistance to phagocytosis and immune lysis, and is presumably necessary for successful infection [1437].

Another mode of evasion is available to nanorobots resident in locations where components of the immune system cannot easily reach. Some pathogens persist on the luminal surfaces of the gastrointestinal tract, the urinary tract, the oral cavity, or in the lumen of salivary gland, the mammary gland, or kidney tubule [1437]. If there is no host cell destruction, the pathogen may avoid inducing an inflammatory response because sensitized lymphocytes or circulating antibodies cannot reach the site to eliminate the infection. Secretory IgA could react with surface antigens on bacterial cells, but the complement sequence would be unlikely to be activated and the cells would not be destroyed [1437].

There are at least two other immunological evasion strategies employed by microbial pathogens or tumor cells. However, these strategies may be inappropriate or inefficient for medical nanorobots, as explained below:

(1) Decoys. The first method is to release surface antigens in soluble form into the surrounding tissue, which can “mop up” antibody before it reaches the bacterial surface. The use of soluble receptors as decoys by the Shope virus [2352] and by the poxviruses [2353-2355] is well known. As another example, soluble bacterial cell wall components are powerful antigens and complement activators that contribute in a major way to the pathology observed in meningitis and pneumonia [1437]. Protein A is produced by S. aureus and is normally bound to the staphylococcal cell surface, but may also be released in a soluble form which can then bind to the Fc region of IgG, thus agglutinating and partially inactivating the IgG [1437]. Malignant tumor cells can release large amounts of MIC, a major histocompatibility class I homolog, which apparently downregulates the NKG2D receptor found on most natural killer cells (NKC) and impairs the action of tumor-specific effector T cells [5684].

(2) Enzymes. The second method is to produce enzymes that destroy antibodies. For instance, body surface-dwelling bacteria [1738] such as Neisseria gonorrhoeae, N. meningitidis, Haemophilus influenzae, Streptococcus pneumoniae, and S. mutans produce IgA proteases that inactivate secretory IgA on mucosal surfaces by cleaving the molecule at the hinge region, detaching the Fc region of the immunoglobulin [1437, 1760]. Candida yeasts display similar activity [1739].

If employed by medical nanorobots, both decoy and enzyme methods would require either onboard storage or manufacturing of protein molecules, thus adding to device complexity. These strategies would also require emissions of active biomolecules into the tissues, an inherently inferior and possibly more dangerous approach compared to methods that involve only surface modifications of the nanodevice.

Immune evasion is much simpler for medical nanorobots once they are inside a cell, since activation of intracellular class II molecules by engineered surfaces is unlikely. A similar trick is already used by many types of microorganisms. In the bacterial world [1605], macrophages infected with Brucella (a coccobacillus), Mycobacterium leprae, or Listeria (a soil saprophyte) support bacterial growth while offering protection from immune responses [1437]. Other intracellular pathogens such as Yersinia (etiologic agent of the plague of the Middle Ages) and Shigella are residents of cells other than phagocytes or other antigen-presenting cells, so their antigens are not displayed on the surface of the infected cell [1437]. Chlamydia pneumoniae can be found inside monocytes [1748] and white blood cells [1749]. Benjamini et al [1760] point out that microbes capable of intracellular survival use several strategies to avoid being killed after phagocytosis: M. tuberculosis and Chlamydia block the fusion of lysosomes with the phagocytic vacuole; H. capsulatum interferes with acidification of the phagolysosomal vacuole; Listeria monocytogenes produces bacterial products that allow it to escape from the phagolysosomal vacuole to the cell cytoplasm (a more nutritionally favorable niche); Shigella flexneri apparently triggers apoptotic death of the phagocytic cell; and Toxoplasma gondii generates its own vacuole to remain isolated from host lysosomes and thus avoids triggering recognition of infected cells by the immune system.

Among the viral pathogens, herpes simplex virus can interfere with immune system recognition of infected cells through a mechanism that inhibits MHC class I molecule presentation on the infected cell and blocks its interaction with virally derived peptides [1760]. Other techniques of immune evasion [2546] employed by viruses include: (1) interference with trafficking along the endocytic pathway; (2) interference with class I MHC biosynthesis in the ER (endoplasmic reticulum); (3) interference with cytosolic proteolysis of viral antigen; (4) diversion of the ER-targeted peptide transporter system; (5) retention and destruction of class I MHC molecules; (6) modification of MHC function after their delivery to the cell surface; (7) blocking transcription of MHC class II proteins; (8) distribution of inhibitory NK receptors at the surface to prevent NK cells from destroying the virus-infected cell; (9) negative cytokine regulation; and even (10) inhibition of apoptosis.

Among the protozoans, intracellular parasites are protected from the immune response while their life cycle is completed inside a cell, resulting in the release of more parasites into the host system along with the death of the host cell. One example is Plasmodium, a protozoan parasite that infects red blood cells and causes malaria. This disease presents in a cyclical fashion [1743], coinciding with the life cycle of millions of parasites that are all in the same life phase simultaneously. Another example is Leishmania, a flagellate protozoan parasite that hides inside macrophages – the macrophage is unable to recognize the parasite within itself and is thus unable to destroy it [1744, 1745]. Other intracellular protozoan parasites include Toxoplasma, Cryptosporidium, and Pneumocystis, which can cause transient or life-threatening illness, some treatable and some not. These examples provide further biological analogs to the nanorobotic cytocarriage (Section 9.4.7) approach.

It is worth noting that the potential ability of nanorobots to hide from the immune system by using variants of the techniques employed by pathogens for similar purpose does not imply that pathogens will correspondingly be able to evade detection by medical nanorobots. It is certainly true that the surfaces of intracellular pathogens that can infect motile phagocytic cells (e.g., the tuberculosis Mycobacterium or the bacterium Listeria, both of which can reside inside macrophages [4588]) are not accessible for direct probing by the antigen sensors of extracellular diagnostic or therapeutic nanorobots. But cell surface markers will usually reveal such infection, so surveillance nanorobots can check for the presence of such markers and thus deny intracellular pathogens a secure hiding place inside human cells. For instance, the membrane surface of macrophages infected by Mycobacterium microti is antigenically different from that of uninfected macrophages [5263]. Listeria-derived peptides are found acting as integral membrane proteins in the plasma membrane of infected macrophages [5264], and other Listeria-infected antigen-presenting cells display hsp60 on their plasma membranes only when infected [5265]. As another example, conserved invariable regions of the antigenic variation protein [5266-5268], of the outer surface proteins [5269-5271], or of other surface-exposed proteins [5272, 5273] of Borrelia can be targeted for detection as reliable pathogenic signatures, by medical nanorobots.

Last updated on 30 April 2004