The adhesion of anti-CD3-activated mouse T cells to syngeneic colon adenocarcinoma cells is differentially regulated by protein tyrosine kinase-, protein kinase C-, and cAMP-dependent pathways in the effector cell

Targeting Adenosine in Cancer Immunotherapy to Enhance T-Cell Function

T cells play a critical role in cancer control, but a range of potent immunosuppressive mechanisms can be upregulated in the tumor microenvironment (TME) to abrogate their activity. While various immunotherapies (IMTs) aiming at re-invigorating the T-cell-mediated anti-tumor response, such as immune checkpoint blockade (ICB), and the adoptive cell transfer (ACT) of natural or gene-engineered ex vivo expanded tumor-specific T cells, have led to unprecedented clinical responses, only a small proportion of cancer patients benefit from these treatments. Important research efforts are thus underway to identify biomarkers of response, as well as to develop personalized combinatorial approaches that can target other inhibitory mechanisms at play in the TME. In recent years, adenosinergic signaling has emerged as a powerful immuno-metabolic checkpoint in tumors. Like several other barriers in the TME, such as the PD-1/PDL-1 axis, CTLA-4, and indoleamine 2,3-dioxygenase (IDO-1), adenosine plays important physiologic roles, but has been co-opted by tumors to promote their growth and impair immunity. Several agents counteracting the adenosine axis have been developed, and pre-clinical studies have demonstrated important anti-tumor activity, alone and in combination with other IMTs including ICB and ACT. Here we review the regulation of adenosine levels and mechanisms by which it promotes tumor growth and broadly suppresses protective immunity, with extra focus on the attenuation of T cell function. Finally, we present an overview of promising pre-clinical and clinical approaches being explored for blocking the adenosine axis for enhanced control of solid tumors.

Commitment of activated T cells to secondary responsiveness is enhanced by signals mediated by cAMP-dependent protein kinase A-I

Modalities that induce specific differentiation to T cell memory in immune responses are important for vaccine design, but there is a paucity of well characterized molecular pathways useful to target for this purpose. We have shown previously that pentoxifylline (PF), a phosphodiesterase (PDE) inhibitor in common clinical use, enhances the commitment of in vitro allo-primed human T cells to secondary responsiveness, a characteristic crucial for memory T cells, which are key determinants of the longevity of the immune response. We now show that this effect can also be mediated by activation of adenylate cyclase (AC) and involves PDE4, but not PDE3 or PDE7. PF-mediated enhancement of T-cell priming is inhibited by blocking AC, is specifically signaled via cAMP-dependent protein kinase A (PKA) isoform I, and is probably independent of both nuclear factor-kappaB and the mitogen-activated protein kinase cascade. Furthermore, known pharmacological inhibitors of AC or PKA by themselves cannot block T-cell priming in the absence of PF or rolipram (Rm), and enhancement of priming requires the presence of PF only relatively late during a 4-day priming in vitro (at 48-96 h), suggesting that pharmacological extension of cAMP-mediated signaling can bring about an event critical for T cell commitment to memory. Furthermore, PF and Rm prevent induction of caspase activation and apoptosis in anti-CD3-activated human T cells. Together, our data suggest that PKA-I-mediated signals triggered by prolonging the half-life of cAMP induced during T-cell priming increase survival of activated T cells and enhance memory T cell commitment.

Adenosine suppresses alpha(4)beta(7) integrin-mediated adhesion of T lymphocytes to colon adenocarcinoma cells

The interaction of T lymphocytes with tumor cells, a key step in the antitumor immune response, is suppressed by adenosine, a nucleoside produced at increased levels within the hypoxic tumor environment. We have explored the mechanism by which adenosine interferes with the lymphocyte:tumor cell interaction. The adhesion of anti-CD3-stimulated T cells to syngeneic MCA-38 mouse colon adenocarcinoma cells did not involve LFA-1 (alpha(L)beta(2)) or VLA-5 (alpha(5)beta(1)). However, antibodies against either lymphocyte alpha(4) or beta(7) (but not beta(1)) integrin subunits, or against VCAM-1 on the tumor cells, significantly suppressed adhesion, showing that the recognition of MCA-38 cells by T cells is strongly dependent upon the association of alpha(4)beta(7) on the effector cells with VCAM-1 on the tumor targets. This association is modulated by adenosine: The ability of adenosine to suppress T cell adhesion to MCA-38 cells was lost if alpha(4)beta(7) was functionally blocked with anti-alpha(4) antibodies (i) prior to or (ii) during the adhesion assay or if (iii) alpha(+)(4) cells were depleted from the T lymphocyte population. The binding of T cells to fibronectin through alpha(4)beta(1) was not suppressed by adenosine. We conclude that adenosine partially inhibits the interaction of T lymphocytes with tumor cells by blocking the function of integrin alpha(4)beta(7).

Renal proximal tubular cell growth and differentiation are differentially modulated by renotropic growth factors and tyrosine kinase inhibitors

BACKGROUND

The renotropic growth factors (GFs), hepatocyte GF (HGF), epidermal GF (EGF), and insulin-like GF-I (IGF-I) accelerate renal regeneration in animal models after toxic or ischemic injury. These GFs initiate their biological effects on renal tubular cells by interaction with specific transmembrane receptor tyrosine kinases.

MATERIALS AND METHODS

In the proximal tubular cell line PT-1, the biological effects of HGF, EGF, and IGF-I and the growth-inhibitory effects of different tyrosine kinase inhibitors (TKIs) were investigated. Receptor binding and tyrosine kinase phosphorylation were determined by ligand binding studies and Western blot analysis.

RESULTS

HGF, EGF, and IGF-I bound with nanomolar affinity to their specific cell membrane receptor tyrosine kinases. In contrast to EGF or IGF-I, HGF induced a variety of cell morphological changes, including cell scattering, formation of tubular structures, and expression of long microvilli on the apical cell membrane. HGF was a 10-fold more potent and more effective growth promoter than EGF or IGF-I. Among the TKIs tested, the mitogenic effect of HGF could be more specifically inhibited by emodin and tyrphostin, that of EGF by methyl-2,5-dihydroxycinnamate, lavendustin A, and genistein, and that of IGF-I by geldanamycin.

CONCLUSIONS

In contrast to EGF and IGF-I, HGF stimulated both growth and differentiation of renal proximal tubular cells, demonstrating the amazing biological potency of this renotropic growth factor. Selective TKIs may be a promising approach to modulate diseases with abnormalities in protein kinase signalling pathways such as renal cell carcinoma.