A single-cell transcriptomic atlas of peripheral blood immune cells spanning progressive canine leishmaniosis.

Dogs play a major role in sustaining the transmission of Leishmania infantum to people, making prevention and treatment of canine leishmaniosis (CanL) public health priorities. However, immune mechanisms underlying progression from subclinical stages to terminal disease in dogs remain ill-defined. To address this gap, we generated a single-cell RNA sequencing map of peripheral immune cells from uninfected and naturally infected dogs across well-defined stages of L. infantum infection. Disease progression was marked by a shift from a lymphoid-to a myeloid-dominated immune landscape. CD4+ T cells transition from naive to effector states, with TH1 cells showing progressive exhaustion signatures paralleling disease progression, while CD8+ T cells exhibited TPEX-like phenotypes with differentiation toward effector and proliferative programs during severe L. infantum infection. Monocytes showed inflammatory remodeling across clinical stages. The LeishDog Atlas provides a framework for understanding immune dysregulation in CanL and a resource for comparative and translational studies of its immunopathology.