Role of NK-Like CD8⁺ T Cells during Asymptomatic Borrelia burgdorferi Infection

Lyme disease (LD) due to Borrelia burgdorferi is the most prevalent vector-borne disease in the United States. There is a poor understanding of how immunity contributes to bacterial control, pathology, or both during LD. Dogs in an area of endemicity were screened for B. burgdorferi and Anaplasma exposure and stratified according to seropositivity, presence of LD symptoms, and doxycycline treatment. Significantly elevated serum interleukin-21 (IL-21) and increased circulating CD3¹ CD94¹ lymphocytes with an NK-like CD8¹ T cell phenotype were predominant in asymptomatic dogs exposed to B. burgdorferi. Both CD94¹ T cells and CD32 CD94¹ lymphocytes, corresponding to NK cells, from symptomatic dogs expressed gamma interferon (IFN-g) at a 3-fold-higher frequency upon stimulation with B. burgdorferi than the same subset among endemic controls. Surface expression of activating receptor NKp46 was reduced on CD94¹ T cells from LD, compared to cells after doxycycline treatment. A higher frequency of NKp46-expressing CD94¹ T cells correlated with significantly increased peripheral blood mononuclear cell (PBMC) cytotoxic activity via calcein release assay. PBMCs from dogs with symptomatic LD showed significantly reduced killing ability compared with endemic control PBMCs. An elevated NK-like CD8¹ T cell response was associated with protection against development of clinical LD, while excess IFN-g was associated with clinical disease.