Guido Frumento

Tryptophan-derived Catabolites Are Responsible for Inhibition of T and Natural Killer Cell Proliferation Induced by Indoleamine 2,3-Dioxygenase

Macrophages exposed to macrophage colony-stimulating factor acquire the capacity to suppress T cell proliferation; this effect is associated with de novo expression of the tryptophan-catabolizing enzyme indoleamine 2,3-dioxygenase (IDO). We have purified IDO and tested its activity in in vitro models of T cell activation. IDO was able to inhibit proliferation of CD4+ T lymphocytes, CD8+ T lymphocytes, and natural killer (NK) cells; proliferation of B lymphocytes was not affected. The inhibitory role of tryptophan and of its catabolites was then tested. In the presence of tryptophan, only l-kynurenine and picolinic acid inhibit cell proliferation. In a tryptophan-free medium cell proliferation was not affected. In the absence of tryptophan inhibition induced by l-kynurenine and picolinic acid was observed at concentrations below the lowest concentration that was effective in the presence of tryptophan, and quinolinic acid acquired some inhibitory capacity. Inhibition of cell proliferation induced by the tryptophan catabolites resulting from IDO activity was selective, applying only to cells undergoing activation. Resting cells were not affected and could subsequently activate normally. We suggest that IDO exerts its effect on cell proliferation by (i) starting the cascade of biochemical reactions that produce the three catabolites and by (ii) enhancing their inhibitory potential by depriving the extracellular microenvironment of tryptophan.

Natural killer cells infiltrating human nonsmall‐cell lung cancer are enriched in CD56brightCD16− cells and display an impaired capability to kill tumor cells

Despite natural killer (NK) cells being originally identified and named because of their ability to kill tumor cells in vitro, only limited information is available on NK cells infiltrating malignant tumors, especially in humans.

Arginase 2 is expressed by human lung cancer, but it neither induces immune suppression, nor affects disease progression

In human prostate cancer, Arginase 2 (ARG2) and nitric oxide synthase (NOS) are concomitantly expressed by tumor cells, and induce tumor immune escape via peroxynitrite‐dependent Tyrosine nitrosylation. Since there were no data regarding this immune suppressive mechanism in other tumor types, and an evaluation of its clinical relevance in human tumors had still to be provided, we have investigated presence and clinical relevance of ARG2 and NOS expression in lung cancer. No evidence of NOS expression was found, no significant NOS enzymatic activity was detected. Instead, ARG2 protein was expressed by tumor cells. In a cohort of 120 patients, the amount of ARG2‐positive tumor cells was significantly higher in small cell lung cancers (SCLC) than in non‐small cell lung cancers (NSCLC). Large cell undifferentiated carcinomas had twice ARG2 than the other NSCLC subtypes. ARG2 expression was increased in Grade 3 tumors, as compared to Grades 1 and 2. However, no relationship was found with tumor size and stage, and with patient survival. Indeed, the enzyme was active, since the Arginine catabolite Ornithine was produced, but Arginine depletion was not attained. In addition, nitrotyrosine was not found in tumor tissue. Accordingly, when tumor cells isolated from lung cancer were incubated with activated autologous T cells, no inhibition of proliferation was detected. Our results indicate that ARG2 is expressed in lung cancer, but it does not induce tumor immune escape and does not affect disease progression, most probably due to the lack of concomitant NOS expression.

Characteristics of the cytosol-synthesized proteins generating class-I-bound peptides.

The proteins synthesized in the cytosol are several thousand, and the number of peptides potentially able to be bound by class I molecules they can generate is therefore huge. On the other hand, the actual number of peptide-class I complexes required for CTL activation is around 200. We focused on the peptides bound by B27 molecules and by the whole class I. By comparing our results with analogous data from other laboratories, we found that 31 peptides matched protein sequences in data bases; in four cases, two peptides are derived from the same protein. The finding of four pairs of identical samples in a sampling of 31 peptides from a pool of unknown magnitude suggests that this pool is quite small. We have estimated the size of this pool by combinatorial analysis and by computer simulation, and we have found a most probable distribution of about 100 to the number of self-proteins that can actually generate peptides bound by class I molecules.