Effect of an empty plasmid containing CpG on dendritic cells: in vitro steady state of stimulation, in vivo prevention of collagen-induced arthritis

Objective

We studied the immune response against pcDNA3.1 in a model of rheumatoid arthritis (RA), namely collagen-induced arthritis (CIA), to understand the role of innate immunity in RA and the inocuity of this empty plasmid used in gene therapy.

Methods

CIA was induced in DBA/1 mice by type II collagen sc injections. At D10 and D24 after immunization, 1 μg pcDNA3.1neo (CpG islets-containing plasmid) or pCor (control plasmid) were injected in lower limbs. DBA/1 BMDC were stimulated in vitro with pcDNA3.1neo, pCor, LPS^+/- TNF for 24 and 48 hours. BMDC were analyzed in flow cytometry, and IL-12, TNF-α, IFN-γ and IL-10 were tested by ELISA in BMDC supernatants. BMDC endocytosis capacity and MLR functional capacity were also studied. CIA was treated with a stimulated dendritic cell population after immunization.

Results

PcDNA3 decreases the incidence of arthritis in collagen immunized DBA mice. Stimulation of BMDC with pcDNA3 increased costimulation molecule expression, cytokine production (IL-12, TNF-α, IFN-γ and IL-10), and allogeneic T cell proliferation, but at a lower level than LPS stimulation. Moreover, pc DNA3 stimulated BMDC have higher endocytosis capacity than LPS stimulated BMDC. Treatment of collagen immunized mice with this steady-state activated-dendritic cell population decreases the incidence of arthritis.

Conclusion

We demonstrated that the stimulation of dendritic cells with an empty plasmid, pcDNA3.1neo, could lead to a steady state of activation that could support tolerance induction. This effect needs to be further characterized and seems to be a 'yes' or 'no' answer, as incidence is decreased but total clinical scores are not statistically modified. Histological studies will help to understand this mechanism.