Reduction in Ophistorchis numbers and Th1 responses: correlation or causation?

This post discusses the following paper, chosen by a classmate for her presentation in a class that was about the pathogenesis of persistent infections:

Kaewraemruaen C, Sermswan RW, Wongratanacheewin S. CpG oligodeoxynucleotides with crude parasite antigens reduce worm recovery in Opisthorchis viverrini infected hamsters. Acta Trop 2016;164:395-401

Opisthorchis viverrini is a food-borne liver trematode, found in South east Asia, giving it the common name - Southeast Asian liver fluke. It has a typical trematode life-cycle (See CDC's page here). Infection itself can be divided into the acute and chronic phases, with the acute phase having a typical Th1 type immune response, characterized by IL12 and IFNg, and the chronic phase having a typical Th2/Treg response with IL4, IL 10 and TGFbeta. Hamsters are used as models to study the immunopathology of this infection.

In a recent paper published from Thailand, Kaewraemruaen et al ¹, in trying to answer the question of the role played by Th1 responses, hypothesize that there might be a protective role played by Th1 cytokines in the early stages of infection. The hypothesis is derived from the fact that in the acute phases of infection, Th1 related cytokines are upregulated, and that hamsters vaccinated against O. viverrini have shown a Th1 skewed response, with low amounts of Treg cytokines IL10 and TGFbeta. In the later stages of infection, the adult parasite skews the response to a Th2/Treg type, and persists in the body, causing the chronic infection.

The methodology used was to induce a Th1 response using an adjuvant that typically induces such a response and determine if the response caused a reduction in the number of adult trematodes recovered after a time period.

Evidence to prove their hypothesis came from the following experiments:

a.  In vitro proliferation of, and IFNg production from mononuclear cells from naïve hamster spleens were higher for the Th1 cytokine inducing adjuvant CpG Oligodeoxynucleotide than the non-CpG oligodeoxynucleotide control and the crude somatic antigen, but lower than the assay positive control (stimulation with PAM and ionomycin) 72 hrs after incubation.

b. Hamsters that had been injected with CpG ODN alone or with crude somatic antigen or somatic antigen alone, had higher numbers of CD4+ T cells, higher amounts of IFNg and higher numbers of IFNg+ T helpers, than the negative controls, although the presence or absence of somatic antigen with the adjuvant had no bearing on the numbers of CD4+ cells.

c. Despite the adjuvant only animals exhibiting robust Th1 responses, number of adult worms recovered from the hamsters was reduced only when the adjuvant was combined with the crude somatic antigen, and this reduction was not statistically significant. However, there were fewer adult worms in animals that received that adjuvant and adjuvant + antigen compared to PBS injected control animals.

The paper is also significant in that the authors used mAbs directed against murine IFNg to cross detect IFNg in hamsters (both IFNg proteins have 56% identity in their primary structures). Also, CpG ODN which acts through TLR 9 in mice to stimulate innate immune cells was able to act on TLR 9 of hamsters, which is expected given the idea of conservation of TLRs across phyla, and the fact that both animals are rodents, and hence closely related.

Thus, the authors sort of show that a Th1 response is protective, as it apparently is in Schistosoma mansoni infections. However, we get into the semantics of what "protection" actually means. My expectation at the beginning of my reading was that the authors meant protection against reinfection, since the Th1 clearly was an acute phase response. For reasons unclear, the conclusion is that a Th1 response causes protection in the acute phase of the disease, with "protection"being defined as a reduction in the number of adult parasites. However from a disease perspective, no histopathology was done to show that the insult to the liver was any different because of the response. Also, it was not shown in any way that the increase in IFNg+ T helpers were specific to the trematodes and were not just randomly activated T helpers found in blood that had little to no impact on the adult trematodes in the liver. The role of other non- Th1 based responses must have also been considered, to explain the reduction in adult parasite numbers.

Kaewraemruaen C, Sermswan RW, Wongratanacheewin S. CpG oligodeoxynucleotides with crude parasite antigens reduce worm recovery in Opisthorchis viverrini infected hamsters. Acta Trop 2016;164:395-401.