Immunomodulators in Bacterial and Fungal Infections: A Review of Their Therapeutic Potential

Proinflammatory Cytokines in the Treatment of Bacterial and Fungal Infections

Mortality due to severe bacterial infections has not been markedly effected by the introduction of new antimicrobial drugs over the last 30-40 years. This has emphasized the need for development of new therapeutic strategies to combat sepsis. The outcome of an infection depends on two factors: the growth of the microorganisms (including the effect of antibacterial drugs), and the host's defensive response to the invading organism. It is known that injection of bacterial products into experimental animals leads to enhanced nonspecific resistance to a variety of microorganisms. The discovery of the specific mediators responsible for modulation of host defense has created new possibilities for the development of alternative treatment strategies. Molecules such as interleukins, interferons, tumor necrosis factors and hematopoietic growth factors have become available in recombinant form, and their therapeutic potential in various infectious diseases has been tested in various experimental models of infections. Initial data in various patient groups indicate that adjunctive therapy with recombinant proinflammatory cytokines may have beneficial effects in the treatment of bacterial and fungal infections.

Th1 and Th2 cytokines in mice with invasive aspergillosis

With a murine model of invasive aspergillosis we investigated cytokine production by CD4+ T helper cells and the effects of cytokine administration or neutralization on the course and outcome of infection. Patterns of susceptibility and resistance to infection were obtained with different strains of mice injected with different inocula of Aspergillus fumigatus conidia. Mice surviving the primary infection also resisted a subsequent lethal infection that was associated with production of gamma interferon by CD4+ T splenocytes. Impaired neutrophil antifungal activity, observed in susceptible mice, was concomitant with a predominant production of interleukin-4 (IL-4) by CD4+ splenocytes. In these mice, exogenous administration of IL-12 failed to induce resistance to infection; in contrast, treatment with soluble IL-4 receptor cured more than 70% of the mice from primary infection and resulted in the onset of acquired resistance to a subsequent lethal infection. These findings indicate that in murine invasive aspergillosis, production of IL-4 by CD4+ T cells may be one major factor discriminating susceptibility and resistance to infection.

Cytokines in the treatment of fungal infections

The incidence of invasive fungal infections in the immunocompromized host has increased during the past decade. Even the recently developed antifungal drugs are unable to cure these infections in patients with severely impaired host defense mechanisms. Cytokines have great potential to augment host resistance and as adjunctive therapy of invasive mycoses. We discuss the mechanisms of host defense against invasive candidiasis, aspergillosis, and cryptococcosis, and review the use of cytokines and growth factors in this setting. Interleukin-1 has been shown effective in an animal model of disseminated candidiasis, even during severe granulocytopenia. Interferon-gamma has been very effective as a modulator of resistance against a variety of fungal infections in vitro. The effect of interferon-gamma against disseminated candidiasis has been demonstrated in a mouse model. Activation of neutrophils is the main mechanism by which interferon-gamma enhances the elimination of Candida, and consequently the agent is not effective in severely granulocytopenic animals. Data on the role of colony-stimulating factors against fungal pathogens are accumulating, and trials with these agents for hematologic patients with invasive fungal infections are now being performed.