Arginase I is constitutively expressed in human granulocytes and participates in fungicidal activity

The balance of arginine metabolism via nitric oxide synthase (NOS) or arginase is an important determinant of the inflammatory response of murine macrophages and dendritic cells. Here we analyzed the expression of the isoform arginase I in human myeloid cells. Using healthy donors and patients with arginase I deficiency, we found that in human leukocytes arginase I is constitutively expressed only in granulocytes and is not modulated by a variety of proinflammatory and anti-inflammatory stimuli in vitro. We demonstrate that arginase I is localized in azurophil granules of neutrophils and constitutes a novel antimicrobial effector pathway, likely through arginine depletion in the phagolysosome. Our findings demonstrate important differences between murine and human leukocytes with respect to regulation and function of arginine metabolism via arginase

Cytotoxicity of tumor antigen specific human T cells is unimpaired by arginine depletion.

Tumor-growth is often associated with the expansion of myeloid derived suppressor cells that lead to local or systemic arginine depletion via the enzyme arginase. It is generally assumed that this arginine deficiency induces a global shut-down of T cell activation with ensuing tumor immune escape. While the impact of arginine depletion on polyclonal T cell proliferation and cytokine secretion is well documented, its influence on chemotaxis, cytotoxicity and antigen specific activation of human T cells has not been demonstrated so far. We show here that chemotaxis and early calcium signaling of human T cells are unimpaired in the absence of arginine. We then analyzed CD8(+) T cell activation in a tumor peptide as well as a viral peptide antigen specific system: (i) CD8(+) T cells with specificity against the MART-1aa26-35*A27L tumor antigen expanded with in vitro generated dendritic cells, and (ii) clonal CMV pp65aa495-503 specific T cells and T cells retrovirally transduced with a CMV pp65aa495-503 specific T cell receptor were analyzed. Our data demonstrate that human CD8(+) T cell antigen specific cytotoxicity and perforin secretion are completely preserved in the absence of arginine, while antigen specific proliferation as well as IFN-γ and granzyme B secretion are severely compromised. These novel results highlight the complexity of antigen specific T cell activation and demonstrate that human T cells can preserve important activation-induced effector functions in the context of arginine deficiency

Human eosinophil granulocytes do not express the enzyme arginase.

Human polymorphonuclear PMN constitutively express the enzyme arginase I, which hydrolyzes arginine to ornithine and urea. This arginine consumption has been recognized as a key pathway of myeloid cell-mediated suppression of the adaptive immune system during inflammation, infection, and tumor growth. Eos granulocytes are crucial immunoregulatory and effector cells of allergic inflammation and infections with parasites and helminths and in a variety of tumors. Here, we analyzed if human Eos also express arginase with its potential immunosuppressive consequences. We show that human peripheral blood Eos do not express arginase I or II protein or arginase enzymatic activity. Correspondingly, no metabolism of arginine to ornithine can be detected in Eos-S. Neither Eos apoptosis nor cytokine-mediated cellular activation induces arginase in human Eos in vitro. Finally, we show that arginase activity and protein are also undetectable in Eos of allergic patients from peripheral blood or from BALF activated in vivo during allergic pulmonary inflammation. This work demonstrates a fundamental difference between neutrophil and Eos granulocytes. As Eos are not equipped with the immunosuppressive enzyme arginase, they cannot participate, via arginine limitation, in the suppression of the evolving adaptive immune response in allergy, infections, or tumor immunity