earlier conflicting data by demonstrating that while CD137-CD137L interactions inhibit myelopoiesis during steady-state conditions they increase myelopoiesis during infection.Keywords: BM r CD137 r Infection r Myelopoiesis r T cellsAdditional supporting information may be found in the online version of this article at the publisher's web-site Introduction CD137 (TNFRSF9, 4-1BB, induced by lymphocyte activation, ILA) is a member of the tumor necrosis factor (TNF) receptor familyCorrespondence: Dr. Herbert Schwarz e-mail: phssh@nus.edu.sg [1,2]. CD137 is highly expressed by T cells following activation, and crosslinking of CD137 delivers potent costimulatory signals to T cells [3]. CD137 agonists enhance T-cell activity and enable the rejection of even established tumors in mice [4,5]. Stimulation of CD137 also enhances protective immune responses against pathogen infections. Agonistic anti-CD137 Abs have been shown to enhance the efficacy of vaccines against Influenza and poxvirus [6,7]. Inclusion of a CD137 ligand-expressing vector in a vaccine, or engineered expression Eur. J. Immunol. 2013Immunol. . 43: 1555Immunol. -1567 of CD137 ligand on monocytes enhanced their ability to induce anti-HIV responses [8,9]. The importance of the CD137 receptorligand system in limiting and clearing viral infections is further testified by the fact that CD137L −/− mice have impaired immunity against lymphocytic choriomeningitis virus [10] and several Influenza virus strains [11,12]. Therefore, the induction of CD137 expression on immune cells in the course of infections can be regarded as part of the anti-pathogen immune response. CD137 ligand (CD137L) is expressed by antigen-presenting cells (APCs), where it serves to costimulate CD137-expressing, activated T cells. The CD137 receptor-ligand system has bidirectional signaling, which is also found at other members of the TNF receptor-ligand families [13]. Bidirectional signaling is possible because CD137L, just like CD137, is expressed as a transmembrane protein on the cell surface and transmits a signal into the cell it is expressed on, a process referred to as reverse signaling [14]. For APCs the CD137L signal is activating. CD137L signaling enhances the activities of all APCs, and thereby contributes to the elimination of the pathogen [15].However, CD137L signaling may enhance the anti-pathogen response by an additional mechanism, i.e. by the induction of myelopoiesis resulting in the generation of more myeloid cells that can enhance the strength and efficiency of the anti-pathogen immune response. This hypothesis is based on our recent findings that the CD137 receptor-ligand system not only regulates the activities of mature immune cells but also induces proliferation of hematopoietic progenitor cells, and their differentiation to myeloid cells, in particular to macrophages [16,17]. However, it has also been reported that CD137−/− and CD137L −/− mice have higher numbers of myeloid cells in the BM than WT mice suggesting an inhibitory effect of CD137 on myelopoiesis [18]. ...