Tuberculosis remains a global health problem, with an estimated 10 million cases and 1.4 million deaths worldwide in 2019 [1]. Extrapulmonary tuberculosis accounts for about 16% of all tuberculosis cases, and it is estimated that 5% to 6% of cases are tuberculous meningitis (TBM) [2]. The global burden of TBM is estimated to be ≥100,000 cases per year [3]. TBM is the most lethal form of tuberculosis, resulting in death or severe disability in approximately 50% of affected patients due to devastating neurological sequelae such as tuberculoma formation, cranial nerve dysfunction, hydrocephalus, and vascular complications including stroke [4]. In contrast, most patients with aseptic viral meningitis (VM) experience a self-limiting clinical course without any neurological sequelae. Therefore, rapid diagnosis of TBM followed promptly by optimal treatment is critical, but diagnosing TBM is often challenging due to delayed or inconclusive laboratory results because of the low sensitivity and slow speed of conventional bacteriology tests. Similar cerebrospinal fluid (CSF) profiles between VM and TBM and an indolent clinical onset with nonspecific initial symptoms add to the challenge of differentiating these conditions [5].

The complement system is a major component of innate immunity and enhances antibody-triggered responses [6]. Although the complement pathway in tuberculosis infection is not fully understood, pathogens in Mycobacterium activate the complement pathway [7]. However, the association of the complement system with TBM has rarely been evaluated, and data on clinical implications are scarce [8].

Due to the clinical ambiguity in the differential diagnosis of TBM, as well as insufficient clinical data on the role of the complement system in TBM, we aimed to evaluate the association between serum complement levels and TBM compared to other meningitis subtypes. Moreover, we aimed to better understand the TBM mortality rate according to serum complement levels.

In this study, we evaluated the associations between serum complement levels and subtypes of infectious meningitis, including TBM, BM, and VM. Both serum C3 and C4 levels were significantly lower in the TBM group than in the VM group in both the total and the propensity score-matched populations. Patients with lower serum C4 level were more likely to have a diagnosis of TBM than of VM. Further, TBM patients with lower serum C3 level had a significantly higher 90-day mortality rate compared to TBM patients with higher serum C3 level.

The role of the complement system in tuberculosis infection has been comprehensively evaluated in many studies. The complement system plays a role in the pathogenesis of tuberculosis by opsonizing M. tuberculosis with specific C3 cleavage products for phagocytosis into alveolar macrophages [12]. The classical pathway is initiated by C1q, which binds to antibody-M. tuberculosis antigen complexes during the adaptive immune response. The binding of C1q results in cleavage of C4 and C2 to the C3 convertase (C4bC2b), which cleaves C3 to produce the opsonin C3b and promote formation of the membrane attack complex, resulting in target cell lysis [7,13,14]. The lectin-binding pathway is activated by recognition of complex sugar moieties on the M. tuberculosis cell surface [7], and an alternative pathway is activated by C3 convertase (C3bBb) and regulated by properdin and factor H. All three complement pathways lead to activation of C3 convertases and production of C3b, which opsonizes M. tuberculosis [12,13].

The alternative complement pathway is relatively unusual in the lungs compared to the classical pathway. Some researchers argue that the classical pathway plays a more active role in the pathogenesis of tuberculosis infection [12], a suggestion which was supported by the increased serum C1q level noted in active tuberculosis infection [15], as well as by the identification of complement pathway gene activation in HIV-1–infected patients with early tuberculosis infection [16]. In our study, reductions in both serum C3 and C4 levels indicate an overactive complement system with increased complement consumption in the classical pathway, which is in line with prior studies revealing the predominant role of the classical pathway in tuberculosis infection. Also, in the formation of a tuberculous granuloma in the central nervous system, the innate immune system and specific T and B lymphocytes play a critical role [3,17]. The complement system is a major component of innate immunity and enhances antibody-triggered responses.

Serum complement level could decrease with increased complement consumption by immune complexes. Representative examples of hypocomplementemia are systemic lupus erythematosus, rheumatoid arthritis, antiphospholipid syndrome, cryoglobulinemia, and vasculitis, such as anti-neutrophil antibody-associated vasculitis. Moreover, infectious diseases with large antigenic loads, such as viral infections (e.g., by hepatitis B and C viruses, parvovirus, and flavivirus), can lead to transient hypocomplementemia as antibodies combine with viral antigens [18].

Many studies evaluating the complement system in the CSF revealed massive complement system activation in patients with BM [19-24]. However, only a few studies included blood complement measurements and correlated results with disease severity and outcomes in BM [19,25,26]. One study demonstrated a significant association between high level of C3 in serum negative for meningococcal antigens [25]. Two other studies also offered negative findings. In our study, there were no significant associations between serum complement levels and BM compared to other meningitis subtypes. As with BM, data on serum complement levels in TBM are scarce, and studies including serial blood sampling to determine complement activation profiles in acute TBM are lacking. In a study that included 23 children with TBM and 24 with non-TBM meningitis, serum complement was evaluated as a potential surrogate marker in the diagnosis of TBM, but no significant results were obtained [8]. However, our study included a much larger study population, with 97 cases of TBM and 588 cases of non-TBM meningitis, and revealed a significant decrease in serum C3 and C4 levels in TBM compared to in VM.

Delays in TBM treatment have been associated with high mortality and morbidity rates as well as complications such as vision loss, cranial nerve dysfunction, hydrocephalus, and vascular complications including stroke and aneurysmal formation and rupture [27,28]. This study provides additional information for differentially diagnosing TBM from VM, which share similar initial clinical characteristics and CSF profiles but have contrasting clinical outcomes.

We observed a significantly increased mortality rate in TBM patients with low initial C3 level. This might be due to an overactive complement system with increased complement consumption, which could result in an uncontrolled inflammatory response and unfavorable outcomes. This is why the complement system is often regarded as a double-edged sword [29] and why complement-targeted drugs, such as eculizumab, have been developed for use in the treatment of BM [30,31].

Our study has several limitations. First, because it was a retrospective study with unbalanced numbers in the groups owing to differences in the natural prevalence of cases, there could be selection bias. To compensate for uneven patient allocation between groups and the potential risk of selection bias, we applied propensity score-matched scoring at a 1:1 ratio to eliminate the effects of selection bias and confounding variables. Second, although there was a clear significant difference in complement levels between TBM and VM patients, the average serum complement values in both groups were within the normal reference ranges. Only patients in the lowest-quartile ranges of C3 and C4 could be referred to as having hypocomplementemia. Third, the small proportion of definite TBM cases was another limitation in our study, but it also reflects the nature of clinical practice in Korea, where the sensitivity of the various bacteriology tests for TBM is very low. The positive rate of cultures in clinical practice is believed to be much lower than that measured in the literature for various reasons [32]. In addition to the paucibacillary nature of CSF in TBM diagnosis, we supposed that there is a possibility that the serous type of TBM, which is characterized by signs and symptoms of mild meningitis with spontaneous recovery, could also complicate diagnosis of TBM. There was a single case of VM with a positive CSF tuberculosis culture and PCR result in our study. This patient was diagnosed with VM because they recovered spontaneously with mild symptoms, and their CSF profile suggested VM. Whether the national bacillus Calmette-Guérin (BCG) vaccine inoculation project in Korea could influence the low sensitivity of the bacteriology tests in TBM is another hypothesis we would like to consider in future research. Finally, detailed clinical data, including those concerning symptom duration, neurologic deficits, headache, neck stiffness, and nausea/vomiting, which are crucial factors in the differentiation of meningitis, were not provided in this study.

This study is the largest to date to evaluate serum complement levels in TBM versus other infectious meningitis subtypes. Significantly decreased serum C3 and C4 levels were observed in TBM compared to VM. We hope our results provide useful information for differentially diagnosing TBM from VM such that timely optimal treatment for TBM can be administered to improve outcomes. Further studies will be required to confirm our findings.