Identification of protective peptides of Fasciola hepatica-derived cathepsin L1 (FhCL1) in vaccinated sheep by a linear B-cell epitope mapping approach

Abstract

Background Fasciolosis is one of the most important parasitic diseases of livestock. The need for better control strategies gave rise to the identification of various vaccine candidates. The recombinant form of a member of the cysteine protease family, cathepsin L1 of Fasciola hepatica (FhCL1) has been a vaccine target for the past few decades since it has been shown to behave as an immunodominant antigen. However, when FhCL1 was used as vaccine, it has been observed to elicit significant protection in some trials, whereas no protection was provided in others. Methods In order to improve vaccine development strategy, we conducted a linear B-cell epitope mapping of FhCL1 in sheep vaccinated with FhCL1, FhHDM, FhLAP and FhPrx plus Montanide and with significant reduction of the fluke burden, sheep vaccinated with FhCL1, FhHDM, FhLAP and FhPrx plus aluminium hydroxide and with non-significant reduction of the fluke burden, and in unvaccinated-infected sheep. Results Our study showed that the pattern and dynamic of peptide recognition varied noticeably between both vaccinated groups, and that the regions 55–63 and 77–84, which are within the propeptide, and regions 102–114 and 265–273 of FhCL1 were specifically recognised only by vaccinated sheep with significant reduction of the fluke burden. In addition, these animals also showed significant production of specific IgG2, whereas none was observed in vaccinated-Aluminium hydroxide and in infected control animals. Conclusions We have identified 42 residues of FhCL1 that contributed to protective immunity against infection with F. hepatica in sheep. Our results provide indications in relation to key aspects of the immune response. Given the variable outcomes of vaccination trials conducted in ruminants to date, this study adds new insights to improve strategies of vaccine development.

Efficacy of a multivalent vaccine against Fasciola hepatica infection in sheep

“…These results observed in group 1 agree with previous studies where high levels of antibodies were related with protection against Fasciola hepatica in ruminants [16,21,37,44]. Furthermore, it was reported in sheep immunised with the same vaccine formulation of group 1, the recognition of different CL1 overlapping peptides from those observed in group 2 and 3, supporting the hypothesis of induction of immune protection with the combination of Montanide and cocktail antigen [24]. Similar results to those reported in sheep were also observed in cattle [49].…”

“…These divergences between the level of protection reported and our results might be attributed to major differences in the vaccine formulation used, which may have led to the development of contrasting results in terms of vaccine efficacy. For instance, we recently reported significant B cell-based immune response differences in sheep when the same F. hepatica antigens were administered with different adjuvants, which resulted in diverse level of protection [ 24 ].…”

“…Some of these vaccination trials have been conducted using only one antigen, a mixture of two antigens or, in recent years, a cocktail of molecules composed by more than two antigens mixed with adjuvants to help boost the immune system [15][16][17][18][19]. Nevertheless, results have shown variable results in terms of protection [15,[20][21][22][23][24] and consequently, to date, there is no vaccine formulation sufficiently efficient to reach commercial development. This is probably because Fasciola hepatica immunomodulates the host's immune response toward a non-protective profile [25], even from early stages of the infection [14,[25][26][27].…”

Differences in the antibody response to adult Fasciola hepatica excretory/secretory products in experimentally and naturally infected cattle and sheep

“…Mapping the four epitopes recognised by experimentally infected animals onto a 3D model of the CL1 protein showed that these eptitopes are located on the surface of the protein and away from the active site, indicating they are unlikely to prevent functional activity of the enzyme. Other studies have identified similar epitopes on CL1, using computational software ( Cornelissen et al, 1999 ), phage display of random 12-mers peptides using rabbit anti- CL1/CL2 sera ( Villa-Mancera et al, 2008 , 2011 ; Villa-Mancera and Mendez-Mendoza, 2012 ; Villa-Mancera et al, 2014 ), or using peptide ELISAs and sera from experimentally infected cattle and sheep as part of vaccine trials ( Garza-Cuartero et al, 2018 ; Buffoni et al, 2020 ). The epitopes identified in our study were in similar regions ( Villa-Mancera et al, 2008 , 2011 ; Villa-Mancera and Mendez-Mendoza, 2012 ; Villa-Mancera et al, 2014 ) but differed to epitopes identified in other studies ( Garza-Cuartero et al, 2018 ; Buffoni et al, 2020 ).…”

Regulation of the Fasciola hepatica newly excysted juvenile cathepsin L3 (FhCL3) by its propeptide: a proposed ‘clamp-like’ mechanism of binding and inhibition

“…It may also represent a parasite strategy to avoid a host immune response that could prevent peptidase activation. Indeed, recently Buffoni et al [ 38 ] have shown that immune responses in animals protected by vaccination with FhCL1 were directed to peptides that span the junction between propeptide and the mature domain of the peptidase.…”