Single-cell RNA-sequencing of leprosy skin lesions reveals a titin (TTN) expressing T cell subset associated with the progressive disease

The immune response to intracellular bacteria is critical for host defense against the invading pathogen. This interaction can be studied in the human disease leprosy caused by the intracellular bacterium Mycobacterium leprae affecting primarily the skin compartments. Patients with tuberculoid leprosy (T-lep) express a Th1 cytokine pattern in skin lesions and develop a protective immunity that eliminates the infection, whereas those with lepromatous leprosy (L-lep) express Th2 cytokines in lesions and sustain a progressive infection. In the reversal reaction (RR) state, patients upgrade, either spontaneously or during chemotherapy, from the L-lep to the T-lep pole. Herein, we used unbiased single-cell RNA-sequencing to explore the breadth of potential immune cell states in leprosy. Unexpectedly, we identified a lymphocyte cluster marked by expression of titin (TTN), which is commonly found in the sarcomere of muscle cells. TTN+ T cells were predominantly enriched in L-lep lesions and RNA-FISH showed a higher number of TTN mRNA dots on CD3+ T cells from L-lep in comparison with RR patients. TTN gene expression decreased in the skin lesion cells of L-lep patients undergoing RR episodes associated with the onset of therapy. Protein expression was confirmed by analysis of T cell clones isolated from leprosy lesions and blood-derived CD3+ T cells from healthy donors. Indeed, immunohistochemical staining of lesions and healthy tonsil showed that TTN was present in the CD3+ T cell zones. Canonical pathways analysis revealed that TTN might be involved with T cell inhibitory RhoA signaling pathway. The role of TTN on T cells remains unknown, however, it could represent a novel mechanism in the skin immune response against cutaneous pathogens.