Characterization of Toll-like receptor associated responses and alternatively activated macrophage associated functions in the immunopathogenesis of neurocysticercosis
Neurocysticercosis (NCC) is one of the most common parasitic diseases of the central nervous system (CNS) caused by the metacestode of the helminth parasite Taenia solium. Its clinical symptoms include headache, seizures, stroke, and hydrocephalus that can have devastating life-long effects. In some cases it is fatal. Evidence suggests that the development and severity of clinical manifestations are associated with the intensity of the immunological response caused by degenerating larvae. Thus, treatments with immunosuppressive/antiinflammatory factors such as corticosteroids are used to control the host inflammatory response and associated neuropathology. However, their long-term use can lead to problematic side effects. To characterize the basis of parasite-induced inflammation elicited the role of Toll-Like Receptors (TLRs), which have been implicated as key regulators of host inflammatory responses, were tested. For these studies we used a murine model of NCC in which mice were intracranially (i.c.) inoculated with a related parasite Mesocestoides corti. Previous work from our laboratory in addition to the present study have shown that expression of all of the known TLRs, except TLR5, are differentially expressed among nervous tissue and infiltrating immune cells during infection. As myeloid differentiation primary-response protein 88 (MyD88) is an adaptor molecule necessary for signal transduction originating from most of the TLRs, we hypothesized that MyD88 would play an important role in CNS inflammation and affect disease outcome in murine NCC. Our results show that infected MyD88 -/- mice display reduced neurological sequelae and improved survival compared to wild-type (WT) mice consistent with decreased astrogliotic reactions and microgliosis as well as reduced infiltrating immune cells. In correlation, pro-inflammatory cytokine levels in the CNS of MyD88-/- mice were significantly lower than their WT infected counterparts. Collectively, these findings suggest that MyD88 plays a prominent role in the development of injurious inflammatory responses that contribute to neuropathology and disease severity in NCC.
To characterize potential upstream PRRs involved in the MyD88 associated responses, we choose to study infection in TLR2 deficient mice since TLR2 is the first TLR to be upregulated in murine NCC and is abundantly expressed through-out the infection process. Parasite infected TLR2-/- mice exhibited a decrease in the number of infiltrating leukocytes and reduced levels of inflammatory mediators in the CNS compared to WT mice. But to our surprise, in contrast to the MyD88-/- NCC mice, TLR2 -/- mice displayed increased parasite growth and neuropathology, and succumbed to infection more quickly than WT or MyD88-/- mice. It is well known that alternatively activated macrophages (AAMs) are usually induced during helminth infection and are thought to play a profound regulatory role especially wound healing effects. Thus, we hypothesize that TLR2 is involved in the generation of AAMs and that TLR2-/- mice would exhibit reduced number of the regulatory cells. To test this, the number of AAMs were analyzed in the brains of TLR2-/-, MyD88-/-, and WT mice. Using the macrophage/microglia marker (CD11b+) together with the AAM associated molecules YM1, 'found in inflammatory zone-1' antigen (Fizz1) and Arginase-1. Interestingly, MyD88-/- mice displayed a significant increase in the CNS levels of YM1, Fizz1, and ARG-1, where as TLR2 -/- NCC mice exhibited a decreased expression of these molecules as compared to WT. The results suggest that TLR2 dependent responses influence the development of AAMs that are important for controlling the disease severity of NCC. As TLR2 responses are thought to be MyD88 dependent, the results suggest either indirect effect of TLR2 or a previously undescribed signaling pathway for TLR2. To further substantiate a protective role of AAMs in murine NCC, we infected mice deficient in STAT6 (signal transducer and activator of transcription 6), a central positive regulator of AAM functions. Infection of STAT6 -/- mice resulted in a significantly reduced accumulation of AAMs compared with infected WT mice and correlated with higher parasite burdens, greater neuropathology and shorter survival time in infected STAT6 -/- mice.
In summary findings from the present study demonstrate that TLRs are upregulated in the CNS of parasite infected mice suggesting an important role for these molecules in NCC. Indeed, MyD88, a central regulator for TLR associated responses, plays a prominent role in the development of inflammatory responses known to contribute to neuropathology and disease severity in NCC. Furthermore, substantial number of macrophages in NCC brains exhibit AAM phenotypes during infection that appears to be protective reducing disease severity. Lastly, our findings suggest a novel role for TLR2 in regulating AAM associated responses in parasite infected brains.