Platelet-leucocyte interactions drive MMP-mediated tissue damage in tuberculosis

Tuberculosis causes inflammation and excess matrix metalloproteinase (MMP) activity which lead to tissue damage and adverse patient outcomes. Platelets are emerging as key drivers of inflammation, and platelet-leucocyte aggregate formation via interactions between platelet P-selectin and monocyte PSGL-1 receptors may regulate tissue destruction in tuberculosis. First, a platelet-monocyte co-culture model was utilised to assess platelet-leucocyte interactions. We then examined M.tb-infected and control lymph node tissue using immunofluorescence microscopy. Finally, we investigated tuberculosis patients (TB, n = 17), healthy controls (HC, n = 14), and patients undergoing bronchoscopy subsequently classified as TB (n = 10) or respiratory symptomatic (RS, n = 14). Whole blood was collected to quantify platelet aggregation using light transmission aggregometry, and platelet-monocyte aggregates (PMA), platelet-neutrophil aggregates (PNA), and platelet receptor expression using flow cytometry. In M.tb-infected monocytes, addition of platelets significantly increased secretion of MMP-1 and MMP-10 and upregulated mmp1 gene expression 4.7-fold. MMP-1 secretion was also increased by addition of platelet-derived soluble factors, and by monocyte PSGL-1 receptor ligation. We observed abundant platelets in M.tb-infected lymph node tissue, localising to PSGL-1 receptors on monocytic cells, and this was not seen in M.tb-uninfected control tissue from patients with reactive hyperplasia or with lymphoma. Ex vivo platelet aggregation in response to stimulation with platelet agonist ADP (3µM, 10µM, and 30µM) was reduced in patients with TB versus HC. PMA were increased in TB and RS versus HC, while PNA were raised only in TB; platelet receptor expression was unchanged. Platelet P-selectin expression, PMA, and PNA correlated with each other but were independent of platelet expression of GPIIb/IIIa, indicating dissociation from thrombotic pathways. In summary, PSGL-1/P-selectin mediated platelet-leucocyte interactions drive inflammation and secretion of MMPs in pulmonary TB. This identifies platelets as important regulators of tissue-damaging inflammatory responses in tuberculosis. Targeting this pathway represents a potential host-directed therapeutic strategy in tuberculosis, and possibly in other lung diseases.