Arginase‐dependent suppression by CpG‐ODN plus IFA‐induced splenic myeloid CD11b⁺Gr1⁺ cells

Abstract

The ability of synthetic oligodeoxynucleotides containing unmethylated cytosine guanine motifs (CpG-ODN) to induce both stimulatory and counter-regulatory responses offers novel opportunities for using these molecules as immunomodulatory agents in different therapeutic strategies. Here, we investigated the potential of CpG-ODN to activate the arginase (ARG) enzyme in vivo and focused on the consequences of this activation in T-cell proliferation. Challenging mice subcutaneously with CpG-ODN emulsified in incomplete Freund's adjuvant (IFA) induced ARG and reduced T-cell proliferation associated with CD3f chain downregulation. Interestingly, impaired T-cell expansion correlated with elevated levels of CD11b + Gr1 + myeloid cells localized near T-cell areas in the spleen. In addition, purified CD11b + cells obtained from the spleen of CpG-ODN+IFA-treated mice exhibited increased ARG activity and ARG I expression along with an augmented [ 3 H]-L-arginine uptake. CD11b + myeloid cells significantly suppressed T-cell proliferation and CD3f chain expression induced by a polyclonal stimulus. Furthermore, these effects could be recovered by the addition of excess L-arginine or by treatment of CD11b + cells with a specific ARG inhibitor. This study provides a novel evidence that CpG-ODN+IFA are able to induce splenic CD11b + cells with ARG activity, with this population being responsible for the impaired T-cell proliferation observed after the treatment with CpG-ODN+IFA. These results underscore a key role of CpG-ODN on ARG activity in vivo and add support to the growing body of evidence in favor of a counter-regulatory role for CpG-ODN in an immune response.

Expansion of myeloid-derived suppressor cells with arginase activity lasts longer in aged than in young mice after CpG-ODN plus IFA treatment

“…The suppression of T-cell proliferative response exerted by splenic MDSCs from aged CpG-ODN+IFA-treated mice is mediated by arginase activity, as the addition of the arginase inhibitor, nor-NOHA, to the cultures almost completely restored T-cell proliferation. These results correlate with the increased L-arginine uptake in myeloid cells from young CpG-ODN+IFA-treated mice, resulting in the depletion of this amino acid from the cell microenvironment [ 15 ]. However, we cannot discard other mechanisms activated in these cells that could, to a lesser extent, also be responsible for the suppression.…”

“…We have recently reported that the numbers of CD11b + Gr1 + cells were increased in the spleen of young BALB/c mice after a single administration of CpG-ODN+IFA [ 15 ]. With this in mind, we investigated whether CpG-ODN+IFA could induce CD11b + Gr1 + cells expansion in aged mice.…”

“…We have previously shown that the T cell suppressor ability of myeloid cells from young mice after CpG-ODN+IFA treatment was linked to a mechanism based on L-arginine depletion by arginase activity [ 15 ]. We therefore investigated whether arginase activity was induced in splenocytes of aged mice after CpG-ODN+IFA treatment.…”

“…Besides this, in previous work we have shown that CpG-ODN+IFA induces in young mice the expansion of a myeloid cell population with high arginase-1 expression and enzymatic activity, responsible for reduced T-cell proliferation and CD3z chain downregulation in the TCR complex in T-cells [ 15 ].…”

“…We have previously demonstrated that CpG-ODN used as an adjuvant was able to induce a specific immune response in aged mice comparable to that in young mice [ 12 – 14 ]. We have also demonstrated that systemic administration of CpG-ODN emulsified in incomplete Freund's adjuvant (CpG-ODN+IFA) induced transitory expansion of splenic MDSCs with CD11b + Gr1 + phenotype in young mice, which caused suppression of T-cell proliferation mediated by arginase activity [ 15 ]. Thus, CpG-ODN, besides being a powerful immunostimulatory compound, is capable of inducing MDSCs as a counter-regulatory response in young mice.…”