However, the high stimulation levels as induced by the adherent splenic cells from B10.Q.Ncf1*/* mice were PF-01367338 nmr not reached. This indicates that in B10.Q mice also other APC are involved, most likely DC. Since CD11c+ DC do not express Aq in MBQ mice, they cannot be accounted for the T-cell stimulation elicited by adherent splenic cells from these mice. In the absence of CII, no detectable IL-2 was produced (data not shown). Contrary to the whole CII molecule, a peptide with high affinity for the MHC II could be presented to the specific T-cell hybridoma with the same efficiency by adherent splenic cells, regardless of their capacity to produce ROS (Supporting

Information Fig. 3). APC expressing Ap or Aq could present this equally well, as previously described 9. To investigate T-cell responses in immunized mice, IFN-γ ELIspots were performed using draining Proteasome inhibition assay (inguinal) lymph node (LN) cells from 10 days immunized B10.P.Ncf1*/*.MBQ or B10.P.Ncf1*/* mice. T cells from B10.P.Ncf1*/*.MBQ LN produced a higher number of IFN-γ

spots as compared to B10.P.Ncf1*/* mice, indicating that also in vivo T cells can be activated by Ncf1*/* macrophages (Fig. 3B). Similar results were obtained with IL-2 production assays of LN cells restimulated with lathyritic CII (data not shown). Next, we investigated if arthritis could be induced when macrophages are the only Nutlin-3 chemical structure APC that can present the antigen. Arthritis was induced in B10.P.MBQ transgenic mice with different Ncf1 genotypes or littermate B10.P.Ncf1*/* mice. Only B10.P.Ncf1*/*.MBQ mice

developed arthritis (Fig. 4A) with an incidence of 40% (Fig. 4B). Expression of Aq on macrophages thus allowed CII presentation in vivo but deficiency in ROS production was required to sufficiently prime and activate autoreactive T cells. Anti-CII antibody levels were determined in sera from these mice 79 days after immunization (Fig. 4C). No difference was observed between B10.P.Ncf1*/*.MBQ and B10.P.Ncf1*/* mice, suggesting that the MBQ transgene did not allow increased activation of anti-CII B cells. The difference in anti-CII IgG between B10.P.Ncf1*/*.MBQ and B10.P.Ncf1*/+.MBQ and B10.P.Ncf1+/+.MBQ suggests that Ncf1 has a role in determining the threshold of activation of B cells. Here, we show for the first time that in the absence of ROS, macrophages are able to prime naïve T cells in vivo, resulting in development of CIA in mice. These data suggest that macrophages have contact with naïve T cells in an antigen-dependent way, but that in an ROS sufficient situation this interaction results in suppression of activation. A physiological explanation for this phenomenon could be that ROS secreted by antigen presenting macrophages might protect against a continuous and aberrant T-cell activation leading to chronic inflammation.