Infectious Disease Research
Chronic Infectious Diseases
Prof. Garth L. Nicolson
Chronic respiratory diseases, such as chronic asthma, airway inflammation, chronic pneumonia and other respiratory diseases, are known to be associated with chronic infections. For example, mycoplasmal infections are a common cause of upper respiratory infections, and severe asthma is commonly associated with mycoplasmal infections. These are complex diseases of largely unknown etiology, but in many cases chronic infections are to blame. Some infections, such as those caused by certain Mycoplasma and Chlaymdia species that are invasive and are found in respiratory epithelial cells are able to suppress immune responses by suppressing the ability of pulmonary macrophages to ingest and kill these infectious agents. Although we do not know exactly what causes respiratory diseases, there is increasing evidence that in many patients chronic infections, particularly by certain bacteria and viruses, play an important role in these diseases along with genetic predisposition and immune dysfunction.
Although mycoplasmal infections are often associated with Chronic Asthma, the exact role of mycoplasmas in the pathogenesis of Asthma remains unclear. Certain Mycoplasma species are involved in respiratory tract infections associated with airway inflammations, induction of bronchial hyperresponsiveness (BHR) and asthmatic attacks. At a minimum, M. pneumoniae infections can cause worsening of conditions in asthmatic patients, whose attacks are associated with significant and specific immune responses. Mycoplasmas are only one of many agents that can trigger BHR, and other infectious or chemical agents may contribute to the complex disease process. Such infectious agents could be involved in helping to cause the illness, or they can affect patients by serving as cofactors for the illness (not causing illness on their own but serving as important factors in the disease process) or even as opportunistic infections that increase patient morbidity (sickness) and complications associated with the disease (see Nicolson et al., Antimicrobics and Infectious Diseases Newsletter, 1999 ; 17(11):81-88).
We and others have found that Mycoplasma species are commonly present in urogenital infections. For example, mycoplasmal infections were detected in more than 12% of females who presented at gynecological services, and they are associated with acute and nonspecific non-gonococcal urethritis in males but not in asymptomatic controls. This type of microorganism is also a common cause of genital infections in women, and it was detectable in 7% of women with sexually transmitted diseases. Mycoplasmal and other chronic bacteria have been implicated in a wide variety of urogenital diseases, such as pelvic inflammatory disease, infertility, non-gonococcal urethritis (NGU) and other genital infections, pyelonephritis, Reiter's syndrome, and peritonitis. The appearance of various bacterial species in bacterial vaginosis may be a result of pathophysiological alterations of the vaginal ecosystem, and mycoplasmas appear to play an important role in this process. Mycoplasmas are also known to interfere in pregnancy and are thought to be involved in at least 11% of patients with fertility problems.
Some Mycoplasma species, M. fermentans, M. penetrans, and M. pirum, have been implicated as infectious cofactors in HIV-AIDS. Using relatively insensitive techniques all three mycoplasmas have been detected in up to 20% of patients with HIV infections, and serological studies have suggested that the presence of M. penetrans is also associated with HIV infection. Moreover, the incidence of systemic mycoplasmal infections in HIV-AIDS patients is probably much, much higher than previously thought and may approach 80% or more. Most of the older analyses were performed using relatively insensitive techniques, such as serological analysis. Pathogenic Mycoplasma species may influence HIV pathogenesis by specific and direct activation or suppression of the immune system, the production of superantigens with subsequent alterations in immune responses, or by their contribution to the oxidative stress observed in HIV-positive patients. Also, the development of AIDS may increase the susceptibility of HIV-infected patients for coinfection with various Mycoplasma species, such as M. fermentans. This species is able to bind HIV capsid protein gp120 permitting adhesion of HIV virions to the mycoplasma surface. Subsequently the HIV viruses could be transported directly to cells expressing specific receptors. After binding to target cells, mycoplasmas can stimulate host cell activation by releasing certain cytokine mediators, which are known effectors for virus reproduction.
Antigen similarities between the surface components of mycoplasmas and HIV-1 have led to speculation that they use similar mechanisms for cell entry. For example, the HIV-1 gp120 envelope glycoprotein and M. genitalium adhesion proteins share protein sequence homology and also have significant similarity with the binding site proteins. The interactions of microorganisms with certain antigens on host cells could contribute to a number of possible outcomes, including immune cell dysfunction, depletion, cell shift (to other subsets of immune cells), antibody-producing immune cell proliferation, hyperglobulinemia (a state where certain immunoglobulin proteins of the same class are overproduced) and antigen-presenting cell dysfunction (antigen presentation to the immune system is a very important step in immunity). Interestingly, all of these have been observed during the development of HIV-AIDS, and this makes certain microorganisms like mycoplasmas so important to understand this disease process.
Mycoplasmal and chlamydial infections of the heart have been reported in patients with different types of carditis or heart infections. The most common association was with M. pneumoniae or C. pneumoniae infection. Endocarditis and myocarditis associated with mycoplasmal and chlamydial infections appear to be an important cause of death. Direct bacterial invasion of M. pneumoniae into pericardial tissue appears to be more likely to cause pericarditis than autoimmune phenomena. Viral and bacterial (Mycoplasma, Chlamydia and Mycobacterium tuberculosis) infections appear to be common causes of myocarditis and/or pericarditis (infections of the muscle and cell lining of the heart), and this is just beginning to be appreciated by infectious disease specialists.