Mycobacterium tuberculosis pathogen load and associated inflammatory responses define infection severity in HIV-associated TB and interact with age to predict mortality.

BACKGROUND: Sepsis pathobiology is shaped by both pathogen load and host characteristics, yet pathogen load is rarely measurable in clinical cohorts. Severe HIV-associated tuberculosis (TB), a leading cause of critical illness in Africa, provides a unique model in which Mycobacterium tuberculosis bacilli load can be quantified. We investigated how pathogen load and age influence sepsis pathophysiology and mortality in this high-risk population. METHODS: We conducted a prospective cohort study of adults admitted with severe HIV-associated TB. Systemic bacillary load was estimated from multiple routinely collected TB diagnostics. Blood-based sepsis markers were summarised using principal components analysis, and pathophysiological processes were evaluated using directed acyclic graphs and regression modelling. FINDINGS: Among 519 patients with microbiologically confirmed TB, 75 (14.5%) died by day 28. Two major axes of host response were identified: PC1, capturing IL-6, CRP, and procalcitonin; and PC2, reflecting venous lactate, aspartate aminotransferase, and cytopenias. Both strongly correlated with bacilli load (PC1 r=0.46; PC2 r=0.43; both p<0.001) but not with age. Mortality independently associated with infection severity - whether defined by bacilli load (aOR 2.1, 95%CI 1.6-2.9) or by inflammatory axes (PC1 aOR 2.3, 95%CI 1.7-3.1; PC2 aOR 1.9, 95%CI 1.5-2.5) - and with age (aOR 1.84, 95%CI 1.4-2.4). Absolute mortality risk therefore reflected an interaction between pathogen load and age. Hyperlactataemia, metabolic acidosis, kidney dysfunction, and temperature were all correlated with bacilli load, inflammation and mortality (all p<0.001). Older patients showed less respiratory compensation for metabolic acidosis, higher blood glucose at a given lactate level, and attenuated fever response at a given bacilli load. INTERPRETATION: Pathogen load and associated inflammatory responses define infection severity in HIV-associated TB and interact with age to predict mortality. These findings highlight physiological constraints underlying organ dysfunction in sepsis and illustrate how pathogen load and age can illuminate mechanisms of critical illness.