Biased action of the CXCR4-targeting drug plerixafor is essential for its superior hematopoietic stem cell mobilization

Following the FDA-approval of the hematopoietic stem cell (HSC) mobilizer plerixafor, orally available and potent CXCR4 antagonists were pursued. One such proposition was AMD11070, which was orally active and had superior antagonism in vitro; however, it did not appear as effective for HSC mobilization in vivo. Here we show that while AMD11070 acts as a full antagonist, plerixafor acts biased by stimulating β-arrestin recruitment while fully antagonizing G protein. Consequently, while AMD11070 prevents the constitutive receptor internalization, plerixafor allows it and thereby decreases receptor expression. These findings are confirmed by the successful transfer of both ligands' binding sites and action to the related CXCR3 receptor. In vivo, plerixafor exhibits superior HSC mobilization associated with a dramatic reversal of the CXCL12 gradient across the bone marrow endothelium, which is not seen for AMD11070. We propose that the biased action of plerixafor is central for its superior therapeutic effect in HSC mobilization.

The frog skin host-defense peptide frenatin 2.1S enhances recruitment, activation and tumoricidal capacity of NK cells

Frog skin is a source of peptides with various biological properties. Frenatin 2.1S, derived from norepinephrine-stimulated skin secretions of the Orinoco lime tree frog Sphaenorhynchus lacteus, exhibits immunostimulatory effects as demonstrated by the promotion of proinflammatory phenotypes of mononuclear cells in mouse peritoneal cavity and spleen. The aim of this study was to identify the populations of host cells sensitive to the action of frenatin 2.1S in vivo and to study its effects on their functional antitumor capacity. A single injection of frenatin 2.1S (100μg) in BALB/c mice increased the presence of peritoneal CD11c⁺ dendritic cells and CD3⁺ T cells 24h after administration and there was a significant increase in the number of IL-17 and CXCR3 expressing inflammatory T cells. Frenatin 2.1S treatment also increased the number of TNF-α expressing F4/80⁺ proinflammatory M1 macrophages. The most striking finding of the study is the marked increase of the number of peritoneal natural killer (NK) cells following frenatin 2.1S injection. Further, frenatin 2.1S administration led to activation of NK cells as evaluated by increased expression of NKG2D, FasL, CD69 and CD107a. The increased ratio of interferon-γ vs. IL-10 producing NK cells is further indication of the proinflammatory action of frenatin 2.1S. Peptide treatment enhanced the tumoricidal action of peritoneal NK cells on 4T1 mouse mammary carcinoma cells as revealed by the real-time automated monitoring of cell status. Our data demonstrate that frenatin 2.1S promotes activation and cytotoxic capacity of NK cells and should be regarded as a candidate for antitumor immunotherapy.