Genetic variation in potential Giardia vaccine candidates cyst wall protein 2 and α1-giardin.

Giardia is a prevalent intestinal parasitic infection. The trophozoite structural protein a1-giardin (a1-g) and the cyst protein cyst wall protein 2 (CWP2) have shown promise as Giardia vaccine antigen candidates in murine models. The present study assesses the genetic diversity of a1-g and CWP2 between and within assemblages A and B in human clinical isolates. a1-g and CWP2 sequences were acquired from 15 Norwegian isolates by PCR amplification and 20 sequences from German cultured isolates by whole genome sequencing. Sequences were aligned to reference genomes from assemblage A2 and B to identify genetic variance. Genetic diversity was found between assemblage A and B reference sequences for both a1-g (90.8% nucleotide identity) and CWP2 (82.5% nucleotide identity). However, for a1-g, this translated into only 3 amino acid (aa) substitutions, while for CWP2 there were 41 aa substitutions, and also one aa deletion. Genetic diversity within assemblage B was larger; nucleotide identity 92.0% for a1-g and 94.3% for CWP2, than within assemblage A (nucleotide identity 99.0% for a1-g and 99.7% for CWP2). For CWP2, the diversity on both nucleotide and protein level was higher in the C-terminal end. Predicted antigenic epitopes were not affected for a1-g, but partially for CWP2. Despite genetic diversity in a1-g, we found aa sequence, characteristics, and antigenicity to be well preserved. CWP2 showed more aa variance and potential antigenic differences. Several CWP2 antigens might be necessary in a future Giardia vaccine to provide cross protection against both Giardia assemblages infecting humans.