Chlamydia trachomatis persistence in vitro: an overview

Abstract

Chlamydiae growing in target mucosal human epithelial cells in vitro can transition from their normal developmental cycle progression, alternating between infectious but metabolically inactive elementary bodies to metabolically active but noninfectious reticulate bodies (RBs) and back to elementary bodies, into a state of persistence. Persistence in vitro is defined as viable but noncultivable chlamydiae involving morphologically enlarged, aberrant, and nondividing RBs. The condition is reversible, yielding infectious elementary bodies after removal of the inducers, including penicillin, interferon-gamma, iron or nutrient starvation, concomitant herpes infection, or maturation of the host cell into its physiologically differentiated state. All aberrant RB phenotypes are not the same, owing to differing up- or down-regulated chlamydial gene sets and subsequent host responses. Although all persistence-inducing conditions exist in vivo, key questions include (1) whether or not aberrant chlamydial RBs occur in vivo during the alternating acute-silent chronic-acute chlamydial infection scenario that exists in infected patients and animals and (2) whether such aberrant RBs can contribute to prolonged, chronic inflammation, fibrosis, and scarring.

Figure 1

Illustrated stages of the genital C. trachomatis developmental cycle in infected human endometrial epithelial cells. A. Attachment and entry of the small, infectious EB at 1 hpi. B. Early inclusion (8–12 hpi) containing a few larger, metabolically-active RB. C. Larger inclusion at mid-developmental cycle containing many RB and a few intermediate bodies (IB) – transitional forms for maturation into infectious EB. D. Maturing late developmental cycle inclusion containing more infectious EB and some RB. Epithelial cells infected with genital C. trachomatis serovars form only 1 large inclusion. E. An example of multiple (4) inclusions of C. suis (or C. psittaci or C. pneumoniae) per single infected cell. A,B -- Bar = 2 µm at 7,000 × magnification; C,D – Bar = 2 µm at 7,000 × mag., E – Bar = 2 µm at 4,200 × mag.

Figure 2

Penicillin-induced aberrant RB in C. trachomatis serovar E-infected human endometrial epithelial cells cultured in vitro. A. Standard inclusion containing RB and EB in a C. trachomatis-infected HEC-1B cell control at 24 hpi, not exposed to penicillin. B,C. C. trachomatis-infected HEC-1B cells at 36 hpi, previously exposed to Penicillin G (20 µ/ml) at 1 hpi. The RB are swollen, abnormal and non-dividing (RB*). The altered metabolic disconnect between transcription/translation in abnormally enlarged, non-dividing RB and cell envelope component biosynthesis results in excessive shedding of RB outer envelope blebs into the inclusion lumen; these antigen-containing (MOMP, LPS, etc.) blebbed vesicles somehow fuse with or cross the inclusion membrane and emerge into the cytoplasm of the infected host cell (arrowheads). D. The C. trachomatis-infected HEC-1B cells pre-exposed to Penicillin G (20 µ/ml) for 35 hrs, followed by removal of the antibiotic and continued cultivation of the infected cells for an additional 12 hrs. The appearance of miniature bodies is one morphological signal of recovering RB and their reprogrammed attempt to undergo cell division to produce progeny. A,B,C – Bar = 10 µm at 2,900 × mag., D. – Bar is 2 µm at 10,000 × mag.

Figure 3

Asynchronous inclusion development and aberrant RB in C. trachomatis E-infected glandular epithelial cells derived from ductal breast carcinoma MCF-7 cells cultured in vitro. N denotes normal RB and EB in a typical inclusion; P denotes morphologically enlarged, aberrant RB in other inclusions in juxtaposed epithelial cells. Bar = 2 µm at 7,000 × mag.