Ph.D., Michigan State University, 1985
Chlamydia trachomatis is the leading cause of bacterial sexually transmitted infection (STI) in the United States, with an estimated 3-5 million new cases per year, as well as the leading cause of preventable blindness worldwide. Furthermore, chlamydial STIs in 80% of women and 50% of men produce little to no symptoms, and thereby remain undetected and untreated. Unfortunately, untreated infection can lead to tubal infertility and ectopic pregnancy. Although these infections can be detected by screening sexually active individuals, widespread screening is not routinely performed. An effective alternative would be the development of a vaccine. However, traditional approaches to vaccine development have been unsuccessful, simply because we do not yet understand the immunobiology of Chlamydia. My research has been focused on gaining that understanding, such that we might someday be able to prevent chlamydial infection. Chlamydial infection triggers the production of a variety of immunomodulatory molecules, collectively known as cytokines. Interferon-gamma, a cytokine produced in response to infection, induces several antimicrobial mechanisms, including the breakdown of tryptophan by indoleamine dioxygenase (IDO). Activation of IDO rapidly depletes intracellular tryptophan, thereby starving tryptophan-dependent Chlamydia of an amino acid it requires for protein synthesis. While this works effectively in tissue culture, the fact that chlamydial infections are widespread and often asymptomatic suggests that Chlamydia are somehow evading this immunological defense mechanism.
Currently, the goal of my laboratory is to determine how Chlamydia evades destruction by IDO. Although IFN-γ induces IDO activity, cytokines involved in inflammation, IL-1 and TNF-α, can enhance the cell's response to interferon. To understand the nature of this interaction, we have been analyzing changes in IDO gene expression using cells that we transfected with a green fluorescent protein (GFP) gene under the control of the IDO regulatory DNA sequence. Our results indicate that regulation of IDO is multifactorial; three different transcription factors contribute to IDO regulation. Although signal transducer and activator of transcription-1 (STAT-1) is required for activation by interferon, activation of nuclear factor-κB (NF-κB) by pro-inflammatory cytokines is required for synergistic expression of a third key transcription factor, interferon regulatory factor-1 (IRF-1), which is required for full activation of the IDO gene. By understanding the nature of the IDO gene regulation, we have established a framework for identifying potential effects of chlamydial infection on these pathways. In addition, we have shown that these cytokines up-regulate the expression of pro-inflammatory cytokine receptors, and that this increase in receptor expression renders the cells more sensitive to cytokine treatment. These experiments, combined with our knowledge of IDO gene regulation, has painted a clearer picture of the complexity of the interaction between multiple cytokines and the generation of a protective response to Chlamydia.
Another goal has been to assess the effect of intracellular infection on the ability of the infected cell to respond to cytokine signals. The use of 2-color flow cytometry permits the assessment of immune responsiveness in individual infected cells. Using this approach, we have discovered that the engagement of toll-like receptors 2 and 4 by Chlamydia up-regulates receptors of interferon-gamma and other pro-inflammatory cytokines by more than 10-fold. This was unexpected, inasmuch as increased receptor expression should lead to greater IDO activity. However, these cells do not gain increased responsiveness to the respective cytokines. Rather, the signaling pathway used by these receptors appears impaired. Discovering the nature of this impairment is currently underway.