Editorial
Manipulation of autophagy by sexually transmitted infections: new opportunities for intervention

CHLAMYDIA TRACHOMATIS

C. trachomatis is the most common sexually transmitted bacterial pathogen, a major cause of infertility and ectopic pregnancy as well as the leading cause of preventable blindness worldwide⁽²¹⁾. According to the Centers for Disease Control in the United States there was a 5.9% increase in the rate of chlamydial infections from 2014 to 2015, resulting in 478.8 cases per 100,000 people⁽²²⁾. C. trachomatis has a unique biphasic developmental cycle. The extracellular elementary body infects epithelial cells by binding to a heparin-containing surface component and is subsequently internalized⁽²³⁾. It then converts into an intracellular form, the reticulate body, which replicates within inclusion bodies in the host cell cytoplasm. Autophagy-related proteins do not assemble into autophagosomes in response to the inclusion body and so C. trachomatis replication is resistant to autophagy⁽²⁴⁾.

The role of interferon (IFN)-? is pivotal in combating chlamydial infections; its introduction eliminates this bacterium⁽²⁵⁾. IFN-? depletes intracellular tryptophan and iron, induces production of nitric oxide, and activates host autophagy^(8,13,26). Recent data have shown that IFN-? induces the fusion of autophagosomes with chlamydial inclusion bodies via a mechanism involving induction of guanylate binding proteins⁽²⁷⁾.

There remains a still largely unmet need to identify the role of autophagy in a chlamydial genital tract infection. What is the mechanism by which an intracellular chlamydial infection prevents autophagy induction and how can this be reversed? Are women with an inherited or acquired deficit in autophagy induction at increased susceptibility to this infection and its sequelae? Can we identify compounds that can safely induce autophagy and/or IFN-? production in women newly infected with C. trachomatis to prevent harmful consequences of this infection?

CANDIDA ALBICANS

C. albicans is a dimorphic fungus that can exist in either a yeast-like or a filamentous form. It is a harmless commensal organism in about 20% of healthy reproductive age women. Under conditions of a transient local immune suppression or the administration of antibiotics that disrupt the vaginal microbiome, endogenous C. albicans prolif- erates, converts from the yeast to the filamentous form, becomes invasive and causes a symptomatic infection. The mechanism that this organism utilizes to resist immune destruction and to persist in the female genital tract involves autophagy. In response to Candida proliferation in the genital tract, the Th17 class of CD4+ T lymphocytes is activated and the cytokine interleukin (IL)-17a is released. IL-17a recruits neutrophils and macrophages to the genital tract to engulf and destroy the fungal invader⁽²⁸⁾. However, Candida has a cell surface receptor that binds IL-17a and induces autophagy in the yeast⁽²⁹⁾. This results in conversion to the filamentous form and formation of a biofilm that resists engulfment. Once the extracellular infection is resolved and phagocytic cells are no longer present the yeast forms are again produced and recolonize the vagina. Thus, activation of autophagy in C. albicans in response to anti-candidal immunity ensures its survival.

Recurrent vulvovaginal candidiasis (RVVC) is a frustrating consequence of a C. albicans infection in about 5% of infected women. A symptomatic episode is usually resolved with anti-fungal treatment, but the infection returns shortly after treatment cessation. It remains untested whether the interference with autophagy in Candida has the potential to allow the immune system to totally eliminate this organism from the genital tract, and, thus, prevent recurrent infections. An antibody that blocks the IL-17a receptor on yeast and/or introduction of an exogenous compound that activates yeast mTOR and prevents autophagy induction are attractive options that remain to be tested.

HERPESVIRUS

Type I and type 2 herpes simplex virus (HSV) are double stranded DNA viruses that infect the female genital tract. They can cause painful lesions and, in pregnancy, are major inducers of neonatal morbidity and mortality. Their intracellular replication should be inhibited and the virus destroyed by autophagy. However, both HSV 1 and HSV 2 produce a protein, infected cell protein 34.5 (ICP34.5), that inhibits formation of the autophagosome and, thereby, prevents viral elimination⁽³⁰⁾. Hu- man cytomegalovirus (CMV) produces an analogous protein, TRS1, that also inhibits autophagy in CMV-infected cells⁽³¹⁾. Thus, herpesvi- ruses persist within host cells by inhibiting autophagy. The introduction of exogenous agents capable of inducing autophagy in herpesvirus-infected cells appears to be a logical option worthy of evaluation.

HUMAN PAPILLOMAVIRUS

HPV, a double stranded DNA virus, is the etiological agent of cervical carcinoma as well as a causative factor for anal cancer and head and neck malignancies⁽³²⁾. HPV infects terminally differentiated epithelial cells in the female genital tract. The induction of autophagy in these infected cells is essential for the production of amino acids needed for HPV replication as well as for the continued survival of the host cell. The HPV E7 protein has been shown to induce autophagy in epithelial cells. Uninfected epithelial cells that have been transfected with the E7 protein undergo autophagy⁽³³⁾.

The inhibition of autophagy in HPV-infected genital tract epithelial cells to limit the consequences of this infection is an attractive but unexplored area of research. Development of protocols to target HPV-infected cells with agents that inhibit autophagy and/or activate mTOR might be useful adjuncts to conventional treat- ments. In addition, contrary to the situation with C. trachomatis, we predict that women with a reduced capacity for autophagy would be relatively resistant to an HPV infection or development of a cervical malignancy. This possibility is easily amenable to testing.

HUMAN IMMUNODEFICIENCY VIRUS

HIV is a retrovirus that infects monocytes/macrophages and CD4⁺ T lymphocytes in the female genital tract. Like herpesviruses, the intracellular replication of HIV should be disrupted by autophagy. However, unlike herpesviruses, HIV uses autophagy to facilitate its replication in monocytes/macrophages. The HIV gag protein binds to an essential autophagy component, LC3, and facilitates the assembly of HIV within autophagosomes⁽³⁴⁾. A second HIV protein, nef, binds to another autophagy component, Beclin 1, and inhibits the autophagosome from interacting with a lysosome⁽³⁴⁾. A third HIV protein, tat, prevents IFN-? from inducing autophagy in HIV-infected cells⁽³⁵⁾. When HIV infects CD4⁺ T cells the HIV env protein is released into the extracellular milieu where it binds to uninfected CD4⁺ T cells. This re- sults in the induction of autophagy in these cells and their subsequent destruction by apoptosis⁽³⁶⁾. This elimination of uninfected T cells down-regulates anti-HIV immunity and facilitates viral persistence.

Thus, HIV has evolved an amazing repertoire of mechanisms that target autophagy. This suggests that autophagy has a major role in HIV biology. However, focusing on the development of mechanisms to promote autophagy to eliminate HIV in infected monocytes/macrophages and to prevent autophagy in uninfected T cells has received scant research attention.

CONCLUSIONS

Autophagy is a major mechanism to eliminate microorganisms that reside within the cytoplasm of infected cells and to promote effective anti-microbial immune responses. Microorganisms that successfully invade the female genital tract have evolved mechanisms to prevent their autophagy-mediated destruction, utilize autophagy to promote their survival and/or promote their ability to replicate within host cells. The testing of novel protocols to prevent or reverse the microorganism-directed subversion of autophagy remains an under-appreciated area of research. It is our hope that this editorial will facilitate laboratory investigations that result in development of novel means to combat the initiation, development and consequences of sexually transmitted infections.