Clinical and pathological characteristics of the Uganda genotype of Mycobacterium tuberculosis

Abstract

Mycobacterium tuberculosis (Mtb), the aetiological agent of tuberculosis (TB) is the leading infectious cause of death globally. The outcome of Mtb infection is variable and depends on host, bacterial and environmental factors. Mtb has evolved into a number of lineages and sublineages exhibiting phylogeographical population structuring and diverse clinical consequences after infection. The Mtb Uganda genotype is the commonest cause of pulmonary TB (PTB) in Kampala, Uganda. Prominent inherited host factors that determine the outcome of TB are human leukocyte antigens (HLA).

To investigate the impact of Mtb genomic diversity and host HLA allelic variability on the clinical outcome of TB infection in Ugandan patients, the clinical and pathological outcome of Mtb Uganda genotype, and the association between HLA II alleles and PTB due to Mtb were studied. The Uganda genotype was found less frequently in extrapulmonary TB (EPTB) than previously reported in PTB in the same setting (Paper I), and tuberculous lymphadenitis patients infected with Mtb Uganda genotype were significantly less prone to have abdominal lymphadenopathy (Paper IV). This may imply that Mtb Uganda genotype has reduced potential to disseminate.

A study of the evolutionary relationships and worldwide distribution of the spoligotypes of Mtb isolates from Ugandan patients with tuberculous lymphadenitis indicated an ongoing evolution of the Uganda genotype, with Uganda at the center of this evolution (Paper II).

HIV negative patients with pulmonary TB and their genetically related healthy household controls were typed for HLA class II alleles (Paper III). The HLA- DQB1*03:03 allele was significantly less frequent in patients compared to healthy controls suggesting that the HLADQB1*03:03 allele may be associated with resistance to TB.

To establish the cause and pathology of fatal mycobacterial disease, the mycobacteria and pathology associated with fatal TB were studied (Paper IV). One quarter of fatal mycobacterial disease was associated with non tuberculous mycobacteria (NTM). Pleural effusions were significantly associated with Mtb disease compared to NTM infection (Paper IV).

To explore the potential use of the CD4+ and CD8+ T cell immunoprofile to diagnose tuberculous effusion, CD4+ and CD8+ T cells from pleural effusions were characterized. CD4+ T cells were significantly more abundant in individuals with TB, and the CD4+/CD8+T cell ratios were significantly higher in tuberculous pleural effusion compared to non tuberculous effusion, however this significance was lost after adjusting for age and ethnicity. Analysis of pleural fluid for the quantity of CD4+ and CD8+ T cells may be useful for establishing a diagnosis of TB in suspicious cases (Paper V).

In conclusion, this thesis highlights the genetic diversity of Mtb with Mtb Uganda as the predominant genotype in EPTB patients in Uganda. Both NTM and Mtb are associated with fatal mycobacterial disease and the pathology findings are indistinguishable, though NTM are significantly less likely to cause pleural effusion. Mycobacterial genetic diversity together with host HLA variability may have clinical consequences. This can be exploited in designing TB diagnostic, management and prevention strategies.