The lower titer of anti-GPL antibodies and two false-negative cases observed in patients with RA could be explained by the following reasons: (1) impaired antibody responses against the MAC GPL antigen by immunosuppressive therapy, (2) altered immune responses to GPL core potentially governed by human leukocyte antigen genes [16], and/or (3) less severe MAC-PD in the two false-negative cases. This evidence concerns the gene IL31RA and rheumatoid arthritis.