Host Cytokines and Immune Responses in Pregnancy Associated Transmission of Arrested Hookworm Larvae

Abstract

Over one billion people worldwide are infected with blood-feeding intestinal hookworms. The life cycle of A. duodenale (humans) and A. caninum (dogs) includes the propensity for L3 to undergo a temporary state of developmental arrest in the host. In female hosts, tissue-arrested L3 reactivate during pregnancy and are transmitted to neonates through milk. During pregnancy, transforming growth factor (TGF)-β2 is upregulated in the mammary gland. Studies in C. elegans show that TGF-β and insulin-like signaling pathways regulate larval arrest and reactivation. We previously utilized an in vitro assay to demonstrate that recombinant human TGF-β stimulates a feeding response in tissue-arrested A. caninum L3. We hypothesize that host expression of TGF-β and pregnancy hormones such as estrogen and prolactin signal arrested L3 to reactivate.

To facilitate in vivo analyses, we utilized a mouse model of A. caninum infection. Mice were utilized because L3 do not develop into adults but arrest in somatic tissues, and reactivate during the periparturient period. We investigated TGF-β1, TGF-β2 and IGF-1 transcript and serum cytokine profiles during late pregnancy, early lactation and mid-lactation to correlate levels with transmammary transmission of L3 to nursing pups. An in vitro co-culture system was developed to mimic in vivo conditions and assess effects of TGF-β and, estrogen and prolactin on larval reactivation. A. caninum L3 were co-incubated with skeletal muscle or mammary epithelial cells and larval reactivation was measured. Additionally, immune responses were assessed as by measuring serum and transcript levels of IFN-γ and IL-4 in skeletal muscle, mammary gland and spleen to dissect the effects of pregnancy and A. caninum infection in the mouse.

Our findings suggest that host-derived TGF- β1 and IGF-1, but not TGF- β2, are important in reactivation and transmission of arrested A. caninum larvae. Also, a Th2-like response characterized by increased IL-4 transcript levels was observed in skeletal muscle, while a mixed Th1⁄Th2 profile was observed in mammary gland when comparing infection with A. caninum versus pregnancy/lactation in BALB⁄c mice.