Leonid Margolis is chief, Section of Intercellular Interactions, Laboratory of Cellular and Molecular Biophysics, and deputy director, NASA/NIH Center for Three-Dimensional Tissue Culture at the National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892 margolis@helix.nih.gov
The threat posed by infectious agents to human health has increased considerably in recent years in part because of bioterrorism, the emergence of new pathogens, and the spread of antibiotic resistance. These developments have highlighted the need for new antimicrobial treatment and prevention strategies. Here, I suggest that one such strategy may be to use benign microbes, or commensals, as antimicrobial agents, harnessing their ability to skew the cytokine-chemokine network to an anti-infective set-point. A "periodic table" that would list the cytokines and chemokines associated with each microbe might reveal new fundamental mechanisms of microbial pathogenesis and predict which microbes would interfere with one another. It could also provide a new therapeutic approach against pathogenic viruses, a group of infectious agents for which there are currently relatively few treatment options.
Pairing off The human body co-exists with a menagerie of commensal microbes, usually referred to as "normal flora," that includes bacteria on mucosal surfaces and skin and latent viruses in tissue cells. In contrast, pathogenic forms of these and other microorganisms invade the body infrequently but then cause disease by damaging various cells and tissues. Pathogen invasion triggers immune effector mechanisms that are coordinated by a complex network of cytokines and chemokines.
Paradoxically, some microbes have learned how to benefit from cytokine- and chemokine-mediated immune reactions by either exploiting them (for example, replication of certain strains of human immune deficiency virus (HIV) is enhanced in the presence of certain chemokines1,
2,
3) or subverting them (for example, human cytomegalovirus encodes a chemokine receptor that scavenges the chemokine RANTES4). Such strategies allow microbes to create conditions favorable for their own maintenance5. However, different microbes have different cytokine and chemokine requirements for their optimal maintenance, and what is optimal for one microbe may be detrimental for another. Therefore it is conceivable that a microbe preparation that appropriately modulates the cytokine-chemokine network to be detrimental to a pathogen might be used as a treatment (see Fig. 1).
Figure 1. Interaction of microbes in the context of a living tissue.
Microbial invasion modulates a cytokine-chemokine network. The cytokine or chemokine upregulated by a microbe Y can interfere with infection by microbe X. If microbe X is a pathogen and microbe Y is non-pathogenic, this strategy may be used as a potential therapy (for example, using herpesvirus 6 or HTLV-2 against CCR5-using HIV-1 or choriomeningitis virus against hepatitis B virus).
Although such an approach has not yet been tested clinically, several epidemiological observations of viral infection (along with animal and in vitro experiments) indicate that it may be feasible. For example, in mouse lymphocytic choriomeningitis, virus infection abolishes replication of the hepatitis B virus, and this process is mediated by tumor necrosis factor- and interferon- (ref. 6). In vitro, cell infection with human T-cell lymphotrophic virus (HTLV)-1 (ref. 7) or HTLV-2 (ref. 8) inhibits replication of HIV-1 variants that use chemokine (CC motif) receptor-5 (CCR5) for infection. This process is mediated by upregulation of CC chemokines9 for which CCR5 is a natural receptor10,
11. Similarly, infection of explants of human lymphoid tissue with human herpesvirus 6 (a virus that is not linked to any disease in adults) upregulates the CC chemokine RANTES12. Accordingly, co-infection of human lymphoid tissue ex vivo with human herpesvirus 6 and CCR5-utilizing HIV-1 results in suppression of HIV infection12.
Recent epidemiological studies have shown that several other microbes can downregulate HIV in infected individuals. For example, HIV-1 copy number falls during an acute infection with the Gram-negative intracellular bacillus Orienta tsutsugamushi, a causative agent of scrub typhus13. In addition, co-infection with GB virus C is associated with delayed progression of HIV disease14,
15, and HIV replication is transiently suppressed during acute measles in co-infected children16.
Although the mechanisms of interference between O. tsutsugamushi, GB virus C, measles virus, and HIV-1 have not been defined, it is probable that they include perturbations of the cytokine-chemokine network. Furthermore, modulation of this network may affect diseases that are not necessarily linked to particular pathogens. For example, the hygiene hypothesis, which has received substantial recent support, postulates that childhood infections reduce the risk of autoimmune and allergic diseases. According to this hypothesis, the decrease in the incidence of childhood infections in Western countries may be responsible for the increase in the prevalence of these conditions17. Along these lines, previous evidence indicates that measles suppresses atopic dermatitis and nephrotic syndromes18,
19.
Safety issues A key issue is whether these experimental and epidemiological observations can be translated into therapies. More specifically, can a microbe preparation that appropriately modulates the cytokine-chemokine network be used as a therapy against pathogens?
In this regard, the primary concern is safety. Deliberate inoculation of live microorganisms into immunocompromised or sick patients could be risky (and potentially life threatening). This is a particular worry in regard to agents that have not been studied thoroughly enough to guarantee that they are not linked to any disease. (Although attempts to exploit measles therapeutically to treat nephrotic syndrome were reported in the mid-1940s20, the obvious risk linked to this infection is nowadays unacceptable.) Moreover, even a "harmless" microbe may become pathogenic under certain conditions, for example in immunodeficient patients.
One way around this problem (as with vaccines) would be to use inactivated microbes, or a biologically active component isolated from them, to modulate the cytokine network. For example, UV-inactivated measles decreases interleukin-12 production by dendritic cells and suppresses the inflammatory immune response in mice similarly to infectious virus; moreover, specific proteins from the measles virus can mimic this effect21.
It should also be stressed that the use of live microbes is not unprecedented. Live-attenuated viruses and bacteria have been widely used in medicine for vaccination since 1796, when Edward Jenner first used preparations of vaccinia virus against smallpox. Since then, various "live" vaccines have been used and most of them have strong safety records.
New vaccines? The approach outlined here can be viewed as a new type of vaccination that creates (perhaps transient) protection indirectly through modulation of the cytokine network rather than directly through stimulation of antibody production or generation of cytotoxic lymphocytes. It relies on the use of an attenuated microbe not against a pathogenic form of the same or closely related microbe, but against a group of unrelated ones (see "Periodic table of microbes?").
The ability of this approach to create an antimicrobial cytokine milieu for various pathogens may also offer an alternative or adjunct to the more traditional pharmaceutical strategies that target pathogens directly using chemical entities. Moreover, exploiting the exquisite tissue tropism that characterizes most microbes may solve the problem of targeted drug delivery, one of the most difficult in pharmacology. Finally, provided that the protective organisms were not too exotic or difficult to obtain, microbe-based antimicrobials could be less expensive and more accessible than current chemical drugs manufactured by pharmaceutical companies, and thus might become more readily available to patients in developing countries.
In general, cytokine network modulation by microbes in vivo can be considered as a universal language for communication between evolutionarily distant microorganisms. Like any language or sign system, this one is complex and redundant (that is, the same message can be expressed in multiple ways) and should be subjected not only to mathematical but also to linguistic analysis22,
23. Even before researchers can fully understand the language, however, it might nonetheless be exploited to guide novel antimicrobial strategies by using attenuated, harmless, or perhaps even killed microbes to modulate the cytokine-chemokine network. Such an approach may constitute a powerful new way to treat infectious (and particularly viral) disease.
and interferon-
(ref. 
