Tuberculosis of the spine in HIV-negative and positive patients : immune and pathogen-related factors in the local distribution of the disease

Abstract

Tuberculosis of the spine is an ancient disease. Evidence of spine involvement has been seen in Egyptian mummies dating back to 900BC. South Africa is the fourth leading country in having a high prevalence of tuberculosis (TB) of the spine, after China, India and Korea. South Africa also has a high incidence of HIV infection. It is not clear in the literature whether the Human Immunodeficiency Virus (HIV) co-infection affects the distribution of the disease in the spine or causes a different radiologic pattern to that found in HIV-negative patients.

The study was therefore undertaken to compare the extent of the distribution of the disease in HIV -negative and –positive patients, to examine the immune factors that might be responsible for any difference in the distribution of the disease and to evaluate any pathogen-related genetic factors that might be responsible for the TB bacilli settling in the spine.

This was a prospective multi-center study on 61 consecutive patients undergoing surgery for tuberculosis of the spine. The HIV status and blood parameters were measured. The angle of kyphosis and the pattern of the disease on plain radiographs were measured. From Magnetic Resonance Imaging, the number of vertebrae involved, the occurrence of skip lesions, the amount of vertebral body loss and the volume of pus formation were measured. The tissue taken at surgery was sent to Anatomic Pathology for histologic examination, to Immunology to examine the expression of cytokines in the granulomas, to Microbiology to identify and culture the bacteria and for drug resistance testing, and for whole genome sequencing to identify any mutations that may be unique to the spine isolates.

The first set of results showed that 70% of the patients with TB of the spine were HIV-positive, which is much higher than that of a similar study done in this country eight years ago in which there were 40% HIV-positive patients. There was significantly more vertebral bone loss and marginally more pus in HIV-positive patients. This is the direct opposite of what is currently reported in the literature on this subject. These results can be explained based on bone destruction, not only by the bacilli, but mainly by the immune response triggered by the TB and HIV infection. The study also found that non-contiguous lesions occur slightly more frequently in HIV-positive patients.

In the second part of the study, the granulomas were classified according to groups 1-4, where 1&2 were poorly formed granulomas and 3&4 were well formed granulomas. The expression of the cytokine TNF-α in the granulomas was also measured. There were no differences between the HIV status of the patient and the grading of the granulomas. There was also no difference between the HIV status of the patient and the expression of the cytokine TNF-α. Of interest is that the granuloma grades 1&2 had less expression of TNF-α, although this was just marginal and not statistically significant. This finding adds to the number of studies currently investigating host directed therapies in the treatment of TB of the spine.

The last set of data relates to whole genome sequencing (WGS) done at the National Institute for Communicable Diseases in Johannesburg. WGS can identify multidrug resistant strains much more accurately than the phenotypic drug resistance testing methods currently in use. Multidrug-resistant TB occurred in 4.7% of these isolates. The study shows that the Mycobacterium tuberculosis lineages that cause disease in the spine are the same as those causing pulmonary TB in South Africa, with the East-Asia (Beijing) strain dominating at 47%. We could not find any mutations specific to these spine isolates, however some genes of unknown function were detected. It is not certain if they encode for any specific functions that may enable the bacilli to survive in the hypoxic environment of the spine. More work needs to be done on these isolates.