Evaluation of antigen-specific immunoglobulin g responses in pulmonary tuberculosis patients and contacts

Abstract

This study aimed to evaluate the serodiagnostic potential of immunoglobulin G (IgG) responses to Mycobacterium tuberculosis antigens in pulmonary tuberculosis (TB) patients, recent TB contacts with latent TB infection (LTBI), and healthy subjects. Infections were assessed using tuberculin skin tests, QuantiFERON-TB Gold In-Tube tests, drug susceptibility testing, and molecular genotyping of clinical isolates. Serum IgG responses to selective M. tuberculosis antigens, including the 38-kDa and 16-kDa antigens, lipoarabinomannan (LAM), and recombinant early secreted antigen target 6 kDa (ESAT-6) and culture filtrate protein 10 kDa (CFP-10), were determined. We found that the serum IgG responses to all antigens might differentiate between active TB and LTBI, with LAM having the highest diagnostic value (area under the curve [AUC] of 0.7756, P < 0.001). Recurrent TB cases showed significantly higher IgG responses to 38 kDa, CFP-10 (P < 0.01), and LAM (P < 0.05) than new cases, and male patients had higher levels of antigen-specific IgG than females (P < 0.05). Conversely, drug resistance and patient body mass index did not affect IgG responses (P > 0.05). LAM-specific IgG responses differentiated between acid-fast bacillus (AFB) smear-positive and -negative patients (P < 0.01), whereas antigen-specific IgG responses did not vary with the M. tuberculosis genotype (P > 0.05). Significantly higher IgG responses to 38 kDa and 16 kDa were observed in AFB smear-negative patients than in controls. These results suggest that assessment of serum IgG responses to selective purified M. tuberculosis antigens may help improve the diagnosis of active TB, particularly for sputum smear-negative patients or recurrent cases, and these may also help to differentiate between active TB and LTBI.

FIG 1

Enrollment of study participants. In total, 159 active TB patients, 51 TB contacts, and 133 healthy subjects were recruited from Mokpo National Hospital (MNH) and from Severance Hospital (SH). Of these, the genotypes of the infecting M. tuberculosis strains were confirmed in 94 TB patients. Based on QFT-IT tests and TSTs, 26 TB contacts were defined as individuals with LTBI and 54 normal healthy controls were identified.

FIG 2

Antigen-specific IgG responses according to infection state and TB recurrence. (a) TB patients (n = 94) had significantly higher IgG responses to all antigens tested than individuals with LTBI (n = 26) and controls (n = 54), whereas there were no significant differences in IgG responses between individuals with LTBI and controls. (b) M. tuberculosis antigen-specific IgG responses were compared between recurrent TB (n = 36) and new (n = 58) cases. In response to 38 kDa, CFP-10, and LAM, recurrent cases showed significantly higher IgG responses, as compared to new cases. The IgG responses to 38 kDa and LAM also differed between new cases and controls, whereas ESAT-6 and CFP-10 did not differentiate between these groups. The median levels of IgG responses are indicated with horizontal bars in red (*P < 0.05; **P < 0.01; ***P < 0.001).

FIG 3

Antigen-specific IgG responses depending on identification of M. tuberculosis strains by AFB smear tests and M. tuberculosis genotypes. (a) The median IgG response to LAM was significantly higher in patients who showed positive AFB smear test results (AFB+ve, n = 36) than those with negative results (AFB-ve, n = 55). However, all AFB smear-negative patients showed significantly higher IgG responses than controls in response to all antigens tested. (b) The majority of patients (81%) were infected with the M. tuberculosis Beijing strain, but genotype did not affect antigen-specific IgG responses (n = 76 for Beijing strain, n = 18 for non-Beijing strain). The median responses are indicated with horizontal bars in red (*P < 0.05; **P < 0.01; ***P < 0.001).

FIG 4

Effect of gender on antigen-specific IgG responses in TB patients. M. tuberculosis antigen-specific IgG responses were compared between male (n = 64) and female (n = 30) TB patients. In response to all antigens, with the exception of ESAT-6, male patients showed significantly higher IgG responses. The median responses are indicated with horizontal bars in red (*P < 0.05; **P < 0.01; ***P < 0.001).