Alpha1-antitrypsin, an endogenous immunoregulatory molecule: distinction between local and systemic effects on tumor immunology

The acute-phase protein α1-antirypsin (AAT) has recently been suggested to exert beneficial effects in various immune-associated pathologies, such as graftversus-host disease, rheumatoid arthritis, multiple sclerosis and allogeneic transplantation. These effects have been demonstrated to take place through direct immunomodulation of dendritic cells, macrophages, neutrophils and B cells, while allowing, in a yet inexplicable distinction, intact isolated functions of T cells and NK cells. With such a unique discriminatory targeting of immunocytes, AAT drives immune responses simultaneously towards regulation of inflammation and expansion of antigen-specific regulatory T cells (Tregs). Based on this intriguing activity profile, a concern was raised regarding the impact of chronic treatment with human AAT with respect to susceptibility to tumors and to metastatic spread. Tumor development is linked to inflammatory responses in a manner which largely promotes immune evasion. Local innate immunocytes, such as tumor-associated macrophages (TAMs), tumor-associated dendritic cells (TADCs) and the recently appreciated tumor-associated neutrophils (TANs), are considered instrumental in primary tumor survival. This is accomplished in part by the release of growth factors and cytokines, including VEGF, TGFβ, IL-10 and IL-1 receptor antagonist (IL-1Ra), all of which have been demonstrated to be elevated in inflammatory conditions during AAT therapy. While clinical studies report heightened serum AAT levels in patients with a variety of advanced tumors, there is no evidence to point to a particular pro-tumor effect of AAT, and the possibility that its elevation in the blood might represent a meresystemic marker– rather than an accessory to tumor development– is grossly overlooked. Indeed, preclinical studies reveal significant inhibition of tumor development during treatment with AAT, and prolonged follow-up studies of individuals who receive life-long excessive doses of intravenous AAT do not depict a rise in tumor occurrence. Considering the prospect of AAT being indicated for medical indications to individuals with normal levels of the protein, its relation to tumor immunology is of immense importance. We hereby review the current knowledge regarding some possible roles that AAT may play, both locally and systemically, in the delicate and detrimental arena of tumor immunology.