Cryptococcal meningitis is a leading cause of mortality in AIDS patients in the developing world . However, the pathophysiology and natural history of infection with Cryptococcus neoformans remain poorly understood. Acute infections following heavy exposure are reported , but most HIV-related clinical disease is thought to result from reactivation of ‘latent’ pulmonary infection acquired many years earlier . The key predisposition to disease development is HIV-related loss of T-cell immunity . However, other factors influencing either the acquisition of infection, reactivation or subsequent dissemination are unknown.
In Cape Town, South Africa, we have noted that a substantial proportion of patients with cryptococcal meningitis have a history of tuberculosis (TB). Together, these are two leading causes of morbidity and mortality among patients accessing antiretroviral therapy (ART) in Africa  and a possible association between these diseases has previously been noted [6–8]. Although this might simply reflect a shared association with CD4 lymphocytopenia, we hypothesized that TB may directly affect risk of cryptococcal disease via a number of potential mechanisms. We therefore examined this relationship in a cohort of patients enrolling in an ART service in Cape Town [9–11].
Clinical data were collected from consenting patients with approval from the Research Ethics Committee of the University of Cape Town. Data from previous studies of TB  and cryptococcal disease  were available for sequential patients enrolling in this well characterized ART cohort between September 2002 and April 2005. The main outcome was the development of microbiologically confirmed cryptococcal meningitis during the first year of follow-up and the temporal association with a history of previous TB or prevalent TB (the latter defined as TB episodes that were either diagnosed or already being treated at enrollment). TB diagnoses were established as previously described in detail for this cohort . Statistical comparisons were made using the χ 2 or Fisher's exact tests. Odds ratios (ORs) with 95% confidence intervals (CIs) were calculated using logistic regression modeling.
Data were available for 707 patients, of whom 26% were men. The median age was 33 years [interquartile range (IQR) = 28–38]; 52% had stage 3 disease and 28% stage 4 disease. The median baseline CD4 cell count was 97 cells/μl (IQR = 46–157) and viral load 76 803 copies/ml (IQR = 33 167–191 030). Most patients (n = 636; 90%) started ART and, of these, 28 (4%) were lost to follow-up and 58 (9%) died during the first year of ART. Among these losses, very few patients (n = 4) had detectable cryptococcal antigen in serum at baseline and all remained free of cryptococcal disease prior to leaving the program (after a median of 47 days). All those lost to follow-up or who died were therefore assumed not to have cryptococcal meningitis at the end of the first year of the study and all 707 patients were included in the analysis.
A history of pulmonary TB (PTB) was recorded in 45% (n = 308) of the cohort and extrapulmonary TB (EPTB) in 8% (n = 55). These episodes (PTB and EPTB) occurred within 2 years prior to enrollment in 66% (n = 241), between 3 and 5 years in 26% (n = 95) and more than 5 years earlier in 7% (n = 27). Prevalent TB was present in 19% (n = 137). Cryptococcal meningitis developed in 2% (n = 13) of the cohort, a median of 35 days after initiating ART; six of these had a history of previous cryptococcal meningitis, a median of 140 days prior to enrollment.
Of patients who developed cryptococcal meningitis, 85% (n = 11) had a history of TB (all PTB). This preceded the initial episode of cryptococcal meningitis in five of the six relapse cases, and only one patient was on rifampicin at the time of relapse. In univariate analysis, a history of previous TB was associated with the development of cryptococcal meningitis (OR = 4.94; 95% CI = 1.1–22.4, P = 0.039). When restricted to PTB, the association was stronger (OR = 6.87; 95% CI = 1.5–31.2, P = 0.013) and when restricted to PTB within 2 years prior to enrollment, the OR was 8.7 (95% CI = 2.4–32.0, P = 0.001). Prevalent TB was not significantly associated with cryptococcal meningitis (OR = 0.75; 95% CI = 0.16–3.4, P = 0.7). Multivariate analysis revealed that both baseline CD4 cell count and history of PTB within the preceding 2 years were strong and independent predictors of the development of cryptococcal meningitis (PTB within 2 years OR = 6.6, 95% CI = 1.3–32.7, P = 0.02; Table 1).
Despite the common association between CD4 lymphocytopenia and the development of TB and cryptococcal meningitis, our data suggest that a history of PTB within the past 2 years may be an independent risk factor for the subsequent development of cryptococcal meningitis. The observation that prevalent TB episodes were not significantly associated may relate to the fact that any impact on cryptococcal disease risk during the period of follow-up is likely to have been curtailed by the rapid ART-induced restoration of immune function and the resulting protection against cryptococcal meningitis.
A number of possible mechanisms may underlie the observed association between these two diseases. Cryptococcus neoformans is ubiquitous and exposure through inhalation of fungal spores is common [12,13] and yet only approximately 10% of AIDS patients develop cryptococcal disease . This suggests CD4-independent factors may also be important. A shared immunological deficit may allow entry and/or dissemination of both Mycobacterium tuberculosis and Cryptococcus neoformans. Defects in pulmonary innate immune function, for example, may underlie high rates of both TB and cryptococcal disease in gold miners with silicosis . Vitamin D deficiency is associated with defects in the production of cathelicidins by macrophages; these are known to be involved in innate responses to both organisms [15,16]. Posttuberculous lung disease is a strong risk factor for respiratory infections, including low-grade pathogens , and might serve as either a portal of entry for new infection, impair clearance, promote reactivation of latent infection or facilitate its dissemination. Convincing evidence exists for latent cryptococcal infection in animal models  and humans  and reactivation of infections after at least 9 years . Although factors leading to reactivation remain largely unknown, the development of an immunosuppressive phenotype that accompanies active TB  could conceivably promote cryptococcal reactivation. Studies are required to explore these various hypotheses.
Patients enrolling in ART cohorts are subject to survival biases and observed associations may be subject to residual confounding. The specificity of the observed association has not been demonstrated and it is possible that the risk of serious opportunistic diseases other than cryptococcal meningitis may also be increased following episodes of TB. Thus, the observed association requires confirmation in prospective studies. The association was nevertheless robust and the temporal relationship plausible. This finding is supported by data showing that cryptococcal disease is an important cause of late mortality in African patients receiving TB treatment . Not only must clinicians be vigilant to the possibility of cryptococcal disease in patients with a recent history of TB, it also may be appropriate to screen such patients for subclinical disease using cryptococcal antigen tests prior to ART initiation.
J.N.J., E.L.C. and S.D.L. are supported by the Wellcome Trust, London, UK (WT081794 and 074641). R.W. is funded in part by the National Institutes of Health, USA, through a CIPRA grant 1U19AI53217-01 and RO1 grant (A1058736-01A1).
The authors have no conflicts of interest
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