Early recruitment of natural CD4⁺Foxp3⁺ Treg cells by infective larvae determines the outcome of filarial infection

Human helminth infections are synonymous with impaired immune responsiveness indicating suppression of host immunity. Using a permissive murine model of filariasis, Litomosoides sigmodontis infection of inbred mice, we demonstrate rapid recruitment and increased in vivo proliferation of CD4⁺Foxp3⁺ Treg cells upon exposure to infective L3 larvae. Within 7 days post-infection this resulted in an increased percentage of CD4⁺ T cells at the infection site expressing Foxp3. Antibody-mediated depletion of CD25⁺ cells prior to infection to remove pre-existing 'natural' CD4⁺CD25⁺Foxp3⁺ Treg cells, while not affecting initial larval establishment, significantly reduced the number of adult parasites recovered 60 days post-infection. Anti-CD25 pre-treatment also impaired the fecundity of the surviving female parasites, which had reduced numbers of healthy eggs and micro-filaria within their uteri, translating to a reduced level of blood microfilaraemia. Enhanced parasite killing was associated with augmented in vitro production of antigen-specific IL-4, IL-5, IL-13 and IL-10. Thus, upon infection filarial larvae rapidly provoke a CD4⁺Foxp3⁺ Treg-cell response, biasing the initial CD4⁺ T-cell response towards a regulatory phenotype. These CD4⁺Foxp3⁺ Treg cells are predominantly recruited from the 'natural' regulatory pool and act to inhibit protective immunity over the full course of infection.