Comparative immune-profiling of tick vaccine formulations and the role of co-infestation with Babesia microti in Ixodes ricinus-infested BALB/c mice

Abstract

Parasites have evolved a wide variety of mechanisms to evade or manipulate their host’s immune responses. Parasite interactions especially in regards to host cellular responses have been well studied in vivo using pathogen free laboratory rodents (e.g. mice). However, co-infestation (or infection) with multiple pathogens and/or parasites is common in animal hosts (e.g. cattle) under field conditions. Such co-infestations have the potential to modulate the host immune responses, enabling infection/infestation that makes the host more susceptible, as well as confounding the efficacy of treatments (i.e. vaccination).

A greater understanding of the immunological interplay elicited during co-infestation of a model host animal with parasites of economic importance could therefore influence the way that the next generation vaccines are designed. Consequently, a co-infection model with Babesia microti and Ixodes ricinus in BALB/c mice was interrogated in the presence and absence of immunisation with a mock antigen (i.e. ovalbumin) formulated in Freund’s adjuvant. Results show that co-infestation of I. ricinus and B. microti skews the host immune response towards a Th1 immune response with a down-regulation in antigen specific IgE antibodies. Both B- and T lymphocyte data indicated that I. ricinus ticks have a negligible effect on lymph node, spleen and blood lymphocyte subpopulations, possibly due to the immunosuppressive effect of tick saliva. Secondly, mice infested with B. microti only or co-infested had a similar effect on the host immune response, with similar lymphocyte subpopulations being differentially regulated. Additionally, co-infestation resulted in significant up-regulation of IL-10 (Th2 cytokine) and TNF-α (Th1 cytokine) associated with potent anti-inflammatory properties, limiting host immune response to pathogens.

Similarly, the effect of formulation with different adjuvants to enhance host immune responses to vaccination were tested in mice using tick-specific antigens (Bm86 and TC-X) derived from the cattle tick, Rhipicephalus microplus. Mice immunised with a combination of Bm86 formulated with GLA-SE adjuvant had a significantly higher Bm86 specific IgG2a antibody titre and Th1 polarised immune response relative to mice vaccinated with Bm86 formulated with Alum. Furthermore, formulations containing Alum had significantly higher concentrations of IL-2 (in the spleen and lymph node) and TNF-α (in the spleen), indicating a more Th1 polarized immune response overall. The chosen adjuvants had negligible stimulatory effects on the ex vivo lymphocyte subpopulations when using control tissues. However, immunisation with both antigens resulted in significantly higher populations of memory B cells (CD80+) and T regulatory cells (CD3+CD4+CD25+). Both these lymphocyte subpopulations are essential in host immune response and maturation following vaccination. Results from this study therefore indicated that vaccines formulated with both the Bm86 and TC-X antigens produces an optimal immune response compared to vaccines containing only Bm86. In regards to the adjuvants, Alum and GLA-SE, different effects were observed on the host immune maturation for the different antigen combinations.

The study was successful in describing the effect co-infestation of a tick and a tick-borne pathogen has on a model organism and how these simultaneous infestations may influence antigen-specific immune reactions. Furthermore, the study is the first to describe the immunological response in BALB/c mice when immunised with Bm86 antigen formulated with different adjuvants. The work completed in this study is essential to efforts being made to develop a R. microplus vaccine for cattle use, taking into account the effect co-infestation has on vaccine efficacy as well as conducting a preliminary vaccination study.