Why is chronic Lyme borreliosis chronic?

Chronic Lyme borreliosis (CLB) can present not only in different organs but also in different patterns. Although many theories exist about the mechanisms leading to CLB, it is known that viable Borrelia burgdorferi can persist for decades and cause late skin manifestations of acrodermatitis chronica atrophicans (ACA). Thus, the immunopathogenetic findings in ACA can serve as a model for studying the chronic course of Lyme borreliosis. Recent findings indicate that the most important cell for antigen presentation, the epidermal Langerhans cell (LC), is invaded by B. burgdorferi in early Lyme borreliosis. Therefore, LCs were stained immunohistochemically with different markers to investigate their functional activity. Numbers of CD1a+ LCs were reduced in erythema migrans but normal or slightly elevated in ACA. In both diseases there was also a marked downregulation of major histocompatibility complex class II molecules on LCs, as measured by staining of human leukocyte antigen DR. This phenomenon might be a mechanism that protects against the presentation of autoantigens and may be the cause of the impaired capacity of LCs to eliminate B. burgdorferi antigens, thus explaining why CLB is chronic.