Background: We earlier showed that 4-phenylbutyrate (PB) can induce cathelicidin LL-37 expression synergistically with 1,25-dihydroxyvitamin D₃ in a lung epithelial cell line. We aimed to evaluate a therapeutic dose of PB alone or in combination with vitamin D₃ for induction of LL-37 expression in immune cells and enhancement of antimycobacterial activity in monocyte-derived macrophages (MDM). Methods: Healthy volunteers were enrolled in an 8-days open trial with three doses of PB [250 mg (Group-I), 500 mg (Group-II) or 1000 mg (Group-III)] twice daily (b.d.) together with vitamin D₃ {5000 IU once daily (o.d.)}, PB (500 mg b.d.) (Group-IV) or vitamin D₃ (5000 IU o.d.) (Group-V), given orally for 4 days. Blood was collected on day-0, day-4 and day-8; plasma was separated, peripheral blood mononuclear cells (PBMC), non-adherent lymphocytes (NAL) and MDM were cultured. LL-37 transcript in cells and peptide concentrations in supernatant were determined by qPCR and ELISA, respectively. In plasma, 25-hydorxyvitamin D₃ levels were determined by ELISA. MDM-mediated killing of Mycobacterium tuberculosis (Mtb) (H37Rv) was performed by conventional culture method. Results: MDM from Group-II had increased concentration of LL-37 peptide and transcript at day-4, while Group-I showed increased transcript at day-4 and day-8 compared to day-0 (p < 0.05). Both Group-I and -II exhibited higher levels of transcript on day-4 compared to Group-III and Group-V (p < 0.035). Increased induction of peptide was observed in lymphocytes from Group-II on day-4 compared to Group-I and Group-IV (p < 0.05), while Group-IV showed increased levels on day-8 compared to Group-I and Group-III (p < 0.04). Intracellular killing of Mtb on day-4 was significantly increased compared to day-0 in Group-I, -II and -V (p < 0.05). Conclusion: The results demonstrate that 500 mg b.d. PB with 5000 IU o.d. vitamin D₃ is the optimal dose for the induction of LL-37 in macrophages and lymphocytes and intracellular killing of Mtb by macrophages. Hence, this dose has potential application in the treatment of TB and is now being used in a clinical trial of adults with active pulmonary TB (NCT01580007).

Tuberculosis (TB) is one of the leading causes of mortality and morbidity, particularly in developing countries, presenting a major threat to the public health. The currently recommended long term treatment regimen with multiple antibiotics is associated with poor patient compliance, which in turn, may contribute to the emergence of multi-drug resistant TB (MDR-TB). The low global treatment efficacy of MDR-TB has highlighted the necessity to develop novel treatment options. Host-directed therapy (HDT) together with current standard anti-TB treatments, has gained considerable interest, as HDT targets novel host immune mechanisms. These immune mechanisms would otherwise bypass the antibiotic bactericidal targets to kill Mycobacterium tuberculosis (Mtb), which may be mutated to cause antibiotic resistance. Additionally, host-directed therapies against TB have been shown to be associated with reduced lung pathology and improved disease outcome, most likely via the modulation of host immune responses. This review will provide an update of host-directed therapies and their mechanism(s) of action against Mycobacterium tuberculosis.

We have previously shown that 8 weeks’ treatment with phenylbutyrate (PBA) (500mgx2/day) with or without vitamin D3 (vitD3) (5000 IU/day) as host-directed therapy (HDT) accelerated clinical recovery, sputum culture conversion and increased expression of cathelicidin LL-37 by immune cells in a randomized, placebo-controlled trial in adults with pulmonary tuberculosis (TB). In this study we further aimed to examine whether HDT with PBA and vitD3 promoted clinically beneficial immunomodulation to improve treatment outcomes in TB patients. Cytokine concentration was measured in supernatants of peripheral blood mononuclear cells (PBMC) from patients (n = 31/group). Endoplasmic reticulum stress-related genes (GADD34 and XBP1spl) and human beta-defensin-1 (HBD1) gene expression were studied in monocyte-derived-macrophages (MDM) (n = 18/group) from PBMC of patients. Autophagy in MDM (n = 6/group) was evaluated using LC3 expression by confocal microscopy. A significant decline in the concentration of cytokines/chemokines was noted from week 0 to 8 in the PBA-group [TNF-α (β = − 0.34, 95% CI = − 0.68, − 0.003; p = 0.04), CCL11 (β = − 0.19, 95% CI = − 0.36, − 0.03; p = 0.02) and CCL5 (β = − 0.08, 95% CI = − 0.16, 0.002; p = 0.05)] and vitD3-group [(CCL11 (β = − 0.17, 95% CI = − 0.34, − 0.001; p = 0.04), CXCL10 (β = − 0.38, 95% CI = − 0.77, 0.003; p = 0.05) and PDGF-β (β = − 0.16, 95% CI = − 0.31, 0.002; p = 0.05)] compared to placebo. Both PBA- and vitD3-groups showed a decline in XBP1spl mRNA on week 8 (p < 0.03). All treatment groups demonstrated increased LC3 expression in MDM compared to placebo over time (p < 0.037). The use of PBA and vitD3 as adjunct therapy to standard TB treatment promoted favorable immunomodulation to improve treatment outcomes. This trial was retrospectively registered in clinicaltrials.gov, under identifier NCT01580007 .