Pulmonary Hypertension (PH) is a fatal disease where abnormal cells termed “neointima” progressively occlude the pulmonary arteries leading to debilitating right heart failure. PH pathology is characterized by perivascular inflammation, including T cell rich Tertiary Lymphoid Follicles (TLFs). Although perivascular inflammation associates with worse outcomes, preclinical studies implicate T cells as having both pathogenic and protective roles in PH. A repertoire of helper (Th) and regulatory (TReg) T cells shape the immune response by sensing antigen via their T cell Receptors (TCRs) and clonally expanding in response. Current PH therapies do not target neointima or inflammation. Lung transplant remains the only curative therapy. There is a critical unmet need to understand how proliferating T cell lineages and their expressed ligands contribute to neointima development in PH. Here, a mouse model of PH has been developed where human-like perivascular inflammation, including TLFs form alongside occlusive neointimal lesions following chronic intranasal antigen administration. Preliminary data finds that neointima and TLF formation is both T cell and antigen dependent. Ablation of Tregs accelerates neointima formation and implicates a Th17 response. A single cell RNA sequencing “Interactome Map” between T and vascular cells identifies Lymphotoxin (Ltb) as a major T cell- vascular signal. Ltb expression is validated in human PH and Ltb blockade halts neointima and TLF formation in mice. Using bulk and single cell TCR sequencing, the development of neointima formation associates with the growth and suppression of T cell clones responding to antigens. The proposals central hypothesis is neointima development in PH is dependent on a repertoire of pathogenic Cd4+ Th cell clones and the pathogenicity of these clones is mediated by T cell expressed ligands, including Ltb. To test this, three Aims are proposed: (1) Determine the Th cell sub-lineages responsible for pulmonary v