Gangliosides (glycosphingolipids) reduce antibody production by inhibiting B-cell receptor (BCR) signaling. We have shown that a copresentation of gangliosides and polyethylene glycol (PEG) on the same liposomes suppresses anti-PEG IgM production in mice. In addition, we recently observed that pDNA incorporated in PEGylated cationic liposomes (PCLs) induces anti-DNA IgM, which could be a hurdle to the development of efficient gene delivery systems. Therefore, the focus of this study was to determine if the copresentation of gangliosides and DNA on the same PCL would suppress antibody production against DNA. PCLs including DNA induced both anti-PEG IgM production and anti-DNA IgM production. The extent of anti-PEG and anti-DNA IgM production was likely dependent on the immunogenicity of the complexed DNA. Treatment of clodronate-containing liposomes, which causes a depletion of phagocytic cells, suppressed anti-PEG IgM production from PCLs that did not include DNA but failed to suppress anti-PEG IgM production from PCLs that complexed DNA (PCLD). Both anti-PEG IgM and anti-DNA IgM was induced in T-cell-deficient nude mice as well as in normal mice following treatment with PCLs and PCLD, respectively. These results indicate that phagocytic cells contribute to anti-PEG IgM production but not to anti-DNA IgM production, while T-cells do not contribute to any form of antibody production. The copresentation of gangliosides and DNA significantly reduced anti-PEG IgM production but unfortunately did not reduce anti-DNA IgM production. It appears that the immunosuppressive effect of gangliosides, presumably via the CD22 signaling pathway, is limited only to anti-PEG immunity.