Setting Sights on Chlamydia Immunity's Central Paradigm: Can We Hit a Moving Target?

B Cell Presentation of Chlamydia Antigen Selects Out Protective CD4γ13 T Cells: Implications for Genital Tract Tissue-Resident Memory Lymphocyte Clusters

Surveillance and defense of the enormous mucosal interface with the nonsterile world are critical to protecting the host from a wide range of pathogens. Chlamydia trachomatis is an intracellular bacterial pathogen that replicates almost exclusively in the epithelium of the reproductive tract. The fallopian tubes and vagina are poorly suited to surveillance and defense, with limited immune infrastructure positioned near the epithelium. However, a dynamic process during clearing primary infections leaves behind new lymphoid clusters immediately beneath the epithelium. These memory lymphocyte clusters (MLCs) harboring tissue-resident memory (Trm) T cells are presumed to play an important role in protection from subsequent infections. Histologically, human Chlamydia MLCs have prominent B cell populations. We investigated the status of genital tract B cells during C. muridarum infections and the nature of T cells recovered from immune mice using immune B cells as antigen-presenting cells (APCs). These studies revealed a genital tract plasma B cell population and a novel genital tract CD4 T cell subset producing both gamma interferon (IFN-γ) and interleukin-13 (IL-13). A panel of CD4 T cell clones and microarray analysis showed that the molecular fingerprint of CD4γ13 T cells includes a Trm-like transcriptome. Adoptive transfer of a Chlamydia-specific CD4γ13 T cell clone completely prevented oviduct immunopathology without accelerating bacterial clearance. Existence of a CD4γ13 T cell subset provides a plausible explanation for the observation that human peripheral blood mononuclear cell (PBMC) Chlamydia-specific IFN-γ and IL-13 responses predict resistance to reinfection.

Development of a Chlamydia trachomatis vaccine for urogenital infections: novel tools and new strategies point to bright future prospects

INTRODUCTION

The "cloaked" bacterial pathogen that is Chlamydia trachomatis continues to cause sexually transmitted infections (STIs) that adversely affect the health and well-being of children, adolescents and adults globally. The reproductive disease sequelae follow unresolved or untreated chronic or recurrent asymptomatic C.trachomatis infections of the lower female genital tract (FGT) and can include pelvic pain, pelvic inflammatory disease (PID) and ectopic pregnancy. Tubal Factor Infertility (TFI) can also occur since protective and long-term natural immunity to chlamydial infection is incomplete, allowing for ascension of the organism to the upper FGT. Developing countries including the WHO African (8.3 million cases) and South-East Asian regions (7.2 million cases) bear the highest burden of chlamydial STIs.

AREAS COVERED

Genetic advances for Chlamydia have provided tools for transformation (including dendrimer-enabled transformation), lateral gene transfer and chemical mutagenesis. Recent progress in these areas is reviewed with a focus on vaccine development for Chlamydia infections of the female genital tract.

EXPERT COMMENTARY

A vaccine that can elicit immuno-protective responses whilst avoiding adverse immuno-pathologic host responses is required. The current technological advances in chlamydial genetics and proteomics, as well as novel and improved adjuvants and delivery systems, provide new hope that the elusive chlamydial vaccine is an imminent and realistic goal.

Chlamydia trachomatis: the Persistent Pathogen

Chlamydia trachomatis is an obligate intracellular bacterium whose only natural host is humans. Although presenting as asymptomatic in most women, genital tract chlamydial infections are a leading cause of pelvic inflammatory disease, tubal factor infertility, and ectopic pregnancy. C. trachomatis has evolved successful mechanisms to avoid destruction by autophagy and the host immune system and persist within host epithelial cells. The intracellular form of this organism, the reticulate body, can enter into a persistent nonreplicative but viable state under unfavorable conditions. The infectious form of the organism, the elementary body, is again generated when the immune attack subsides. In its persistent form, C. trachomatis ceases to produce its major structural and membrane components, but synthesis of its 60-kDa heat shock protein (hsp60) is greatly upregulated and released from the cell. The immune response to hsp60, perhaps exacerbated by repeated cycles of productive infection and persistence, may promote damage to fallopian tube epithelial cells, scar formation, and tubal occlusion. The chlamydial and human hsp60 proteins are very similar, and hsp60 is one of the first proteins produced by newly formed embryos. Thus, the development of immunity to epitopes in the chlamydial hsp60 that are also present in the corresponding human hsp60 may increase susceptibility to pregnancy failure in infected women. Delineation of host factors that increase the likelihood that C. trachomatis will avoid immune destruction and survive within host epithelial cells and utilization of this knowledge to design individualized preventative and treatment protocols are needed to more effectively combat infections by this persistent pathogen.

IL-4-secreting eosinophils promote endometrial stromal cell proliferation and prevent Chlamydia-induced upper genital tract damage

Genital Chlamydia trachomatis infections in women typically are asymptomatic and do not cause permanent upper genital tract (UGT) damage. Consistent with this presentation, type 2 innate and T_H2 adaptive immune responses associated with dampened inflammation and tissue repair are elicited in the UGT of Chlamydia-infected women. Primary C. trachomatis infection of mice also causes no genital pathology, but unlike women, does not generate Chlamydia-specific T_H2 immunity. Herein, we explored the significance of type 2 innate immunity for restricting UGT tissue damage in Chlamydia-infected mice, and in initial studies intravaginally infected wild-type, IL-10^-/-, IL-4^-/-, and IL-4Rα^-/- mice with low-dose C. trachomatis inoculums. Whereas Chlamydia was comparably cleared in all groups, IL-4^-/- and IL-4Rα^-/- mice displayed endometrial damage not seen in wild-type or IL-10^-/- mice. Congruent with the aberrant tissue repair in mice with deficient IL-4 signaling, we found that IL-4Rα and STAT6 signaling mediated IL-4-induced endometrial stromal cell (ESC) proliferation ex vivo, and that genital administration of an IL-4-expressing adenoviral vector greatly increased in vivo ESC proliferation. Studies with IL-4-IRES-eGFP (4get) reporter mice showed eosinophils were the main IL-4-producing endometrial leukocyte (constitutively and during Chlamydia infection), whereas studies with eosinophil-deficient mice identified this innate immune cell as essential for endometrial repair during Chlamydia infection. Together, our studies reveal IL-4-producing eosinophils stimulate ESC proliferation and prevent Chlamydia-induced endometrial damage. Based on these results, it seems possible that the robust type 2 immunity elicited by Chlamydia infection of human genital tissue may analogously promote repair processes that reduce phenotypic disease expression.