T-Cell Epitopes in Variable Segments ofChlamydia trachomatisMajor Outer Membrane Protein Elicit Serovar-Specific Immune Responses in Infected Humans

Abstract

We previously identified 18 stimulatory Chlamydia trachomatis major outer membrane protein (MOMP) peptides containing at least 23 epitopes presented with various HLA class II allotypes. Only one peptide contained an epitope localized in a variable segment (VS2). Continued studies reported here identified a total of five VS peptides containing T-cell epitopes that are distributed among MOMPs VS1, VS2, and VS4. Only MOMP-primed T-cell cultures from subjects infected with serovar E responded to the serovar E VS peptides, while the response of such cultures to constant-segment peptides was independent of the infecting serovar. Furthermore, MOMP-primed T cells proliferated in response only to the VS peptides encoded in serovar E but not to the corresponding peptides derived from serovar F, I, or J, confirming that these responses were serovar specific.Chlamydia trachomatis causes ocular and genital tract infections worldwide. In the United States, the annual cost of treating sexually transmitted diseases caused by C. trachomatis is billions of dollars (9). Effective preventive measures against C. trachomatis infections are not yet available, due in part to insufficient understanding of human immune responses to C. trachomatis. Evidence for protective immunity against C. trachomatis is scant and inconclusive. However, earlier studies have suggested that short-lived immunity develops after an episode of chlamydial infection (1,10,12) and that protection may be serovar specific (reviewed in reference 8); while reinfection with C. trachomatis is common, it is usually not with serovars of preceding infections. Consequently, many immunological studies of human genital tract infections have been focused on the major outer membrane protein (MOMP) because C. trachomatis serovars are defined by serological variation in this protein. In humans, MOMP elicits C. trachomatisneutralizing antibodies that recognize its surface-exposed variable segments (VSs), where amino acid sequences vary substantially among serovars (4). Therefore, some have speculated that B-cell responses directed toward serovar-specific epitopes located in VSs of MOMP are a critical component of protective immunity (16)(17)(18).Because of the role of T helper (Th) cells in priming and maintaining B-cell responses, we have investigated HLA class II-restricted Th cell responses to MOMP in humans genitally infected with C. trachomatis, focusing on commonly involved serovar E. The main results of our first study (14) were that (i) in vitro priming of peripheral blood mononuclear cells (PBMC) with MOMP yielded MOMP-specific Th cell cultures from 83% of serovar E-infected subjects; (ii) proliferative responses of T-MOMPЈ cultures to synthetic peptides localized at least 23 Th cell epitopes, and T-MOMPЈ cultures from most subjects responded to multiple epitopes; (iii) multiple HLA-DR and other HLA class II molecules were used to present the MOMP epitopes to the Th cells; and (iv) remarkably, 22 of the 23 epitopes were distributed among the five MOMP constant segments ...

Interstrain Gene Transfer inChlamydia trachomatisIn Vitro: Mechanism and Significance

“…A role for spontaneous mutation followed by in vivo selection in the escape of C. trachomatis from human immune defenses is suggested by the high proportion of urogenital tract isolates that have amino acid substitutions in the polymorphic ompA gene, which encodes the major outer membrane protein (MOMP) (3,5,7,11,17,21,27). Humans have diverse B-cell-mediated (33) and T-cell-mediated (16,17,27,28) responses to MOMP. The possibility that at least some of these responses are protective is suggested by the occurrence of mutations in the same ompA segments of multiple clinical isolates (5); some of the mutations alter human T-cell epitopes (27).…”

“…Humans have diverse B-cell-mediated (33) and T-cell-mediated (16,17,27,28) responses to MOMP. The possibility that at least some of these responses are protective is suggested by the occurrence of mutations in the same ompA segments of multiple clinical isolates (5); some of the mutations alter human T-cell epitopes (27). This implication that there is in vivo selection for certain ompA mutants (21) is supported by the fact that the frequencies of spontaneous mutants involving several non-ompA loci in C. trachomatis are Ͻ10 Ϫ7 (2,8) but the frequency of ompA mutants among DNAsequenced clinical isolates is about 10 Ϫ2 to 10 Ϫ1 (22,23); while the unmeasured ompA mutation rate may prove to be above average for C. trachomatis, many MOMP mutants probably have been selected for in vivo.…”

Direct Detection and Magnetic Isolation ofChlamydia trachomatisMajor Outer Membrane Protein-Specific CD8+ CTLs with HLA Class I Tetramers

“…While any of these mutations could be expected to impair the ability of CTLs to recognize the epitope, mutations at residue 156 are especially likely to cause such impairment because it is an anchor residue for peptide binding to HLA-A2. Recurrent mutations at the same residues suggest that there has been in vivo selection for mutants that escaped immune recognition specific for the wild-type epitope; it may be that CTLs specific for the MOMP155-163 epitope ordinarily contribute to restraint of genital tract infections in subjects infected with Ct serovar E. Interestingly, CTL epitope MOMP155-163 largely overlaps two Th epitope-containing peptides (13,14) as well as peptide MOMP139 -163 that may contain a human B cell epitope (47). It must now be determined whether the mutations occurring at residues 155, 156, and 157 functionally disrupt the Ab and Th epitopes as well as the CTL epitope.…”

“…It is not clear how Ct Ags could gain access to either the MHC class I (cytosol) or class II Ag processing (endolysosome) compartments during its infection of epithelial cells. Despite this unknown, we have identified about 30 HLA class II-restricted Th cell epitopes (13,14) and eight HLA class I-restricted CTL epitopes in MOMP (12,30) that are commonly recognized by subjects who have genital tract infections with Ct. These T cells, if present at sites of infection in vivo, could contribute to resolution and/or immunopathology of Ct infection.…”

“…The great majority of Th and CTL epitopes we identified in MOMP are localized in constant domains of MOMP (12,13,30) and could play an important role in providing cross-serovar protection. In light of apparent serovarspecific immunity against Ct, it is of particular interest that we identified several serovar-specific Th and CTL epitopes localized in the variable domains of MOMP (12)(13)(14). T cell responses directed to the variable domains of MOMP, in addition to neutralizing Abs, could contribute to serovar-specific immunity.…”