Effect of Immune Response on Transmission Dynamics for Sexually Transmitted Infections

Reproductive system infections in women: lower genital tract syndromes

Gynecological and obstetrical infectious diseases are an important component of women's health. A system approach to gynecological and obstetrical infection helps unify and classify microbial etiology and pathogenesis within a clinical anatomical framework of lower and upper genital tract syndromes. The reproductive system of women includes the vulva, vagina, cervix, uterus, fallopian tubes and ovaries. During pregnancy, additional tissues include the chorioamnion and placenta together with the fetus and amniotic fluid. We review in two parts reproductive system infection syndromes in women using selected research results to illustrate the clinical utility of the system approach in terms of diagnosis, treatment and prevention. We conclude that a reproductive system perspective will lead to improvements in understanding, management and prevention of these diseases.

Intravaginal Chlamydia trachomatis Challenge Infection Elicits TH1 and TH17 Immune Responses in Mice That Promote Pathogen Clearance and Genital Tract Damage

While ascension of Chlamydia trachomatis into the upper genital tract of women can cause pelvic inflammatory disease and Fallopian tube damage, most infections elicit no symptoms or overt upper genital tract pathology. Consistent with this asymptomatic clinical presentation, genital C. trachomatis infection of women generates robust TH2 immunity. As an animal model that modeled this response would be invaluable for delineating bacterial pathogenesis and human host defenses, herein we explored if pathogen-specific TH2 immunity is similarly elicited by intravaginal (ivag) infection of mice with oculogenital C. trachomatis serovars. Analogous to clinical infection, ascension of primary C. trachomatis infection into the mouse upper genital tract produced no obvious tissue damage. Clearance of ivag challenge infection was mediated by interferon (IFN)-γ-producing CD4+ T cells, while IFN-γ signaling blockade concomitant with a single ivag challenge promoted tissue damage by enhancing Chlamydia-specific TH17 immunity. Likewise, IFN-γ and IL-17 signaling blockade or CD4+ T cell depletion eliminated the genital pathology produced in untreated controls by multiple ivag challenge infections. Conversely, we were unable to detect formation of pathogen-specific TH2 immunity in C. trachomatis-infected mice. Together, our work revealed C. trachomatis infection of mice generates TH1 and TH17 immune responses that promote pathogen clearance and immunopathological tissue damage. Absence of Chlamydia-specific TH2 immunity in these mice newly highlights the need to identify experimental models of C. trachomatis genital infection that more closely recapitulate the human host response.

The arrested immunity hypothesis in an immunoepidemiological model of Chlamydia transmission

For curable infectious diseases, public health strategies such as treatment can effectively shorten an individual's infectious period, and thus limit their role in transmission. However, because treatment effectively eliminates antigen impingement, these types of control strategies may also paradoxically impair the development of adaptive immune responses. For sexually transmitted Chlamydia trachomatis infections, this latter effect has been coined the arrested immunity hypothesis, and is discussed to carry significant epidemiological implications for those individuals who return to similar sexual networks with similar sexual behavior. Here, we examine the effect of antibiotic treatment on the spread of Chlamydia infection through a simple immunoepidemiological framework that characterizes the population as a collection of dynamically evolving individuals in small, paradigmatic networks. Within each individual there is an explicit representation of pathogen replication, accumulation and persistence of an immune response, followed by a gradual waning of that response once the infection is cleared. Individuals are then nested in networks, allowing the variability in the life history of their infection to be functions of both individual immune dynamics as well as their position in the network. Model results suggest that the timing and coverage of treatment are important contributors to the development of immunity and reinfection. In particular, the impact of treatment on the spread of infection between individuals can be beneficial, have no effect, or be deleterious depending on who is treated and when. Although we use sexually transmitted Chlamydia infection as an example, the observed results arise endogenously from a basic model structure, and thus warrant consideration to understanding the interaction of infection, treatment, and spread of other infectious diseases.

Immunobiological outcomes of repeated chlamydial infection from two models of within-host population dynamics

Background

Chlamydia trachomatis is a common human pathogen that mediates disease processes capable of inflicting serious complications on reproduction. Aggressive inflammatory immune responses are thought to not only direct a person's level of immunity but also the potential for immunopathology. With human immunobiology being debated as a cause of prevailing epidemiological trends, we examined some fundamental issues regarding susceptibility to multiple chlamydial infections that could have implications for infection spread. We argue that, compared to less-frequent exposure, frequent exposure to chlamydia may well produce unique immunobiological characteristics that likely to have important clinical and epidemiological implications.

Methods and Results

As a novel tool for studying chlamydia, we applied principles of modeling within-host pathogen dynamics to enable an understanding of some fundamental characteristics of an individual's immunobiology during multiple chlamydial infections. While the models were able to reproduce shorter-term infection kinetics of primary and secondary infections previously observed in animal models, it was also observed that longer periods between initial and second infection may increase an individual's chlamydial load and lengthen their duration of infectiousness. The cessation of short-term repeated exposure did not allow for the formation of long-lasting immunity. However, frequent re-exposure non-intuitively linked the formation of protective immunity, persistent infection, and the potential for immunopathology.

Conclusions

Overall, these results provide interesting insights that should be verified with continued study. Nevertheless, these results appear to raise challenges for current evidence of the development of long-lasting immunity against chlamydia, and suggest the existence of a previously unidentified mechanism for the formation of persistent infection. The obvious next goal is to investigate the qualitative impact of these results on the spread of chlamydia.

A unified framework of immunological and epidemiological dynamics for the spread of viral infections in a simple network-based population

BACKGROUND

The desire to better understand the immuno-biology of infectious diseases as a broader ecological system has motivated the explicit representation of epidemiological processes as a function of immune system dynamics. While several recent and innovative contributions have explored unified models across cellular and organismal domains, and appear well-suited to describing particular aspects of intracellular pathogen infections, these existing immuno-epidemiological models lack representation of certain cellular components and immunological processes needed to adequately characterize the dynamics of some important epidemiological contexts. Here, we complement existing models by presenting an alternate framework of anti-viral immune responses within individual hosts and infection spread across a simple network-based population.

RESULTS

Our compartmental formulation parsimoniously demonstrates a correlation between immune responsiveness, network connectivity, and the natural history of infection in a population. It suggests that an increased disparity between people's ability to respond to an infection, while maintaining an average immune responsiveness rate, may worsen the overall impact of an outbreak within a population. Additionally, varying an individual's network connectivity affects the rate with which the population-wide viral load accumulates, but has little impact on the asymptotic limit in which it approaches. Whilst the clearance of a pathogen in a population will lower viral loads in the short-term, the longer the time until re-infection, the more severe an outbreak is likely to be. Given the eventual likelihood of reinfection, the resulting long-run viral burden after elimination of an infection is negligible compared to the situation in which infection is persistent.

CONCLUSION

Future infectious disease research would benefit by striving to not only continue to understand the properties of an invading microbe, or the body's response to infections, but how these properties, jointly, affect the propagation of an infection throughout a population. These initial results offer a refinement to current immuno-epidemiological modelling methodology, and reinforce how coupling principles of immunology with epidemiology can provide insight into a multi-scaled description of an ecological system. Overall, we anticipate these results to as a further step towards articulating an integrated, more refined epidemiological theory of the reciprocal influences between host-pathogen interactions, epidemiological mixing, and disease spread.

Syphilis in the United States: an update for clinicians with an emphasis on HIV coinfection

Diagnosis and treatment of syphilis are challenging because of its variable clinical presentation and course and the lack of definitive tests of cure after treatment. This review of the most recent literature on the epidemiology, clinical manifestations, current diagnosis, and treatment of syphilis is focused toward clinicians who treat patients with this disease. Syphilis coinfection with human immunodeficiency virus is emphasized because it is increasingly common in the United States and affects the initial presentation, disease course, diagnosis, and treatment of syphilis. Of particular consequence is the effect of human immunodeficiency virus on the clinical diagnosis, prevalence, and course of neurosyphilis, one of the most serious consequences of syphilis infection.