Kelvin P. Lee

Myeloid-derived suppressor cell development is regulated by a STAT/IRF-8 axis

Myeloid-derived suppressor cells (MDSCs) comprise immature myeloid populations produced in diverse pathologies, including neoplasia. Because MDSCs can impair antitumor immunity, these cells have emerged as a significant barrier to cancer therapy. Although much research has focused on how MDSCs promote tumor progression, it remains unclear how MDSCs develop and why the MDSC response is heavily granulocytic. Given that MDSCs are a manifestation of aberrant myelopoiesis, we hypothesized that MDSCs arise from perturbations in the regulation of interferon regulatory factor-8 (IRF-8), an integral transcriptional component of myeloid differentiation and lineage commitment. Overall, we demonstrated that (a) Irf8-deficient mice generated myeloid populations highly homologous to tumor-induced MDSCs with respect to phenotype, function, and gene expression profiles; (b) IRF-8 overexpression in mice attenuated MDSC accumulation and enhanced immunotherapeutic efficacy; (c) the MDSC-inducing factors G-CSF and GM-CSF facilitated IRF-8 downregulation via STAT3- and STAT5-dependent pathways; and (d) IRF-8 levels in MDSCs of breast cancer patients declined with increasing MDSC frequency, implicating IRF-8 as a negative regulator in human MDSC biology. Together, our results reveal a previously unrecognized role for IRF-8 expression in MDSC subset development, which may provide new avenues to target MDSCs in neoplasia.

Sustained antibody responses depend on CD28 function in bone marrow-resident plasma cells.

CD28 signaling is essential for maintenance of long-term antigen-specific antibody production and for persistence of plasma cells in the bone marrow of mice.

Efficacy of lenalidomide in patients with chronic lymphocytic leukemia with high-risk cytogenetics.

Patients with chronic lymphocytic lymphoma (CLL) with high-risk cytogenetics [del(11q)(q22.3) or del(17p)(p13.1)] have limited therapeutic options and their prognosis remains poor. This analysis was conducted to determine the clinical activity of lenalidomide in patients with high-risk disease. Relapsed/refractory patients with CLL enrolled in a phase II clinical trial who had del(11q)(q22.3) or del(17p)(p13.1) were included in this analysis. Patients received single agent lenalidomide for 21 days of the 4 week treatment cycle. The overall response rate among patients with high-risk cytogenetics was 38%, with 19% of patients achieving a complete response. Median progression-free survival was 12.1 months, which is higher than demonstrated with other agents in comparable patient populations. In addition, the estimated 2-year survival probability was 58%, demonstrating that the responses achieved with lenalidomide are durable, even in patients with CLL with high-risk disease with poor risk cytogenetics.

Tumor flare reaction associated with lenalidomide treatment in patients with chronic lymphocytic leukemia predicts clinical response.

In patients with chronic lymphocytic leukemia (CLL), treatment with lenalidomide induces a unique, previously uncharacterized, immune response called tumor flare reaction (TFR). The clinical significance of this reaction remains unknown.

Distribution of Bim determines Mcl-1 dependence or codependence with Bcl-xL/Bcl-2 in Mcl-1-expressing myeloma cells.

Dependence on Bcl-2 proteins is a common feature of cancer cells and provides a therapeutic opportunity. ABT-737 is an antagonist of antiapoptotic Bcl-2 proteins and therefore is a good predictor of Bcl-x(L)/Bcl-2 dependence. Surprisingly, analysis of Mcl-1-dependent multiple myeloma cell lines revealed codependence on Bcl-2/Bcl-x(L) in half the cells tested. Codependence is not predicted by the expression level of antiapoptotic proteins, rather through interactions with Bim. Consistent with these findings, acquired resistance to ABT-737 results in loss of codependence through redistribution of Bim to Mcl-1. Overall, these results suggest that complex interactions, and not simply expression patterns of Bcl-2 proteins, need to be investigated to understand Bcl-2 dependence and how to better use agents, such as ABT-737.

BH3-only proteins Noxa, Bmf, and Bim are necessary for arsenic trioxide-induced cell death in myeloma

The use of arsenic trioxide (ATO) to treat multiple myeloma (MM) is supported by preclinical studies as well as several phase 2 studies, but the precise mechanism(s) of action of ATO has not been completely elucidated. We used gene expression profiling to determine the regulation of apoptosis-related genes by ATO in 4 MM cell lines and then focused on Bcl-2 family genes. ATO induced up-regulation of 3 proapoptotic BH3-only proteins (Noxa, Bmf, and Puma) and down-regulation of 2 antiapoptotic proteins Mcl-1 and Bcl-X(L). Coimmunoprecipitation demonstrated that Noxa and Puma bind Mcl-1 to release Bak and Bim within 6 hours of ATO addition. Bak and Bim are also released from Bcl-X(L). Silencing of Bmf, Noxa, and Bim significantly protected cells from ATO-induced apoptosis, while Puma silencing had no effect. Consistent with a role for Noxa inhibition of Mcl-1, the Bad-mimetic ABT-737 synergized with ATO in the killing of 2 MM lines. Finally, Noxa expression was enhanced by GSH depletion and inhibited by increasing GSH levels in the cells. Understanding the pattern of BH3-only protein response should aid in the rational design of arsenic-containing regimens.

Effect of spaceflight on human stem cell hematopoiesis: suppression of erythropoiesis and myelopoiesis.

Humans subjected to periods of microgravity develop anemia, thrombocytopenia, and abnormalities in red blood cell structure. The causes of these abnormalities are complex and unclear. The in vitro effects of spaceflight on hematopoietic cell proliferation and differentiation were investigated during the space shuttle missions STS‐63 (Discovery) and STS‐69 (Endeavour). CD34+ bone marrow progenitor cells were cultured in liquid suspension culture and on hematopoietic supportive stromal cells using hollow‐fiber culture modules. One set of cultures was maintained at microgravity (flight cultures) for the last 8–10 days of culture and a second control was at full gravity (ground control). Over the 11‐ to 13‐test‐day period, ground control culture total cell number increased 41.0‐ to 65.5‐fold but flight culture total cell number increased only 10.1‐ to 17.6‐fold (57–84% decrease). Comparing ground control cultures and microgravity cultures, respectively, for progenitor cell content, myeloid progenitor cell numbers expanded 2.6‐ to 17.5‐fold compared with 0.9‐ to 7.0‐fold and erythroid progenitor cell numbers expanded 2.0‐ to 4.1‐fold in ground control cultures but actually declined at microgravity (>83% reduction). Moreover, microgravity cultures demonstrated accelerated maturation/differentiation toward the macrophage lineage. These data indicate that spaceflight has a direct effect on hematopoietic progenitor cell proliferation and differentiation and that specific aspects of in vitro hematopoiesis, particularly erythropoiesis, involve gravity‐sensitive components.

Biological effects and clinical significance of lenalidomide-induced tumour flare reaction in patients with chronic lymphocytic leukaemia: in vivo evidence of immune activation and antitumour response.

Lenalidomide has demonstrated impressive antileukaemic effects in patients with chronic lymphocytic leukaemia (CLL). The mechanism(s) by which it mediates these effects remain unclear. Clinically, CLL patients treated with lenalidomide demonstrate an acute inflammatory reaction, the tumour flare reaction that is suggestive of an immune activation phenomenon. Samples from CLL patients treated with lenalidomide were used to evaluate its effect on the tumour cell and components of its microenvironment (immune cellular and cytokine). Lenalidomide was unable to directly induce apoptosis in CLL cells in vitro, however it modulated costimulatory (CD80, CD83, CD86) surface molecules on CLL cells in vitro and in vivo. Concurrently, we demonstrated that NK cell proliferation was induced by lenalidomide treatment in patients and correlated with clinical response. Cytokine analysis showed increase in levels of TNF‐α post‐lenalidomide treatment, consistent with acute inflammatory reaction. Furthermore, the basal cytokine profile (high IL‐8, MIG, IP‐10 and IL‐4 levels and low IL‐5, MIP1a, MIP1b, IL12/p70) was predictive of clinical response to lenalidomide. Collectively, our correlative studies provide further evidence that the antileukaemic effect of lenalidomide in CLL is mediated not only through modulation of the leukaemic clone but also through elements of the tumour microenvironment.

CD28 Expressed on Malignant Plasma Cells Induces a Prosurvival and Immunosuppressive Microenvironment

Interactions between the malignant plasma cells of multiple myeloma and stromal cells within the bone marrow microenvironment are essential for myeloma cell survival, mirroring the same dependence of normal bone marrow-resident long-lived plasma cells on specific marrow niches. These interactions directly transduce prosurvival signals to the myeloma cells and also induce niche production of supportive soluble factors. However, despite their central importance, the specific molecular and cellular components involved remain poorly characterized. We now report that the prototypic T cell costimulatory receptor CD28 is overexpressed on myeloma cells during disease progression and in the poor-prognosis subgroups and plays a previously unrecognized role as a two-way molecular bridge to support myeloid stromal cells in the microenvironment. Engagement by CD28 to its ligand CD80/CD86 on stromal dendritic cell directly transduces a prosurvival signal to myeloma cell, protecting it against chemotherapy and growth factor withdrawal-induced death. Simultaneously, CD28-mediated ligation of CD80/CD86 induces the stromal dendritic cell to produce the prosurvival cytokine IL-6 (involving novel cross-talk with the Notch pathway) and the immunosuppressive enzyme IDO. These findings identify CD28 and CD80/CD86 as important molecular components of the interaction between myeloma cells and the bone marrow microenvironment, point to similar interaction for normal plasma cells, and suggest novel therapeutic strategies to target malignant and pathogenic (e.g., in allergy and autoimmunity) plasma cells.

Calmodulin kinase II regulates the maturation and antigen presentation of human dendritic cells.

Dendritic cells (DC) are professional antigen‐presenting cells, which activate the adaptive immune system. Upon receiving a danger signal, they undergo a maturation process, which increases their antigen presentation capacity, but the responsible regulatory mechanisms remain incompletely understood. A Ca2+‐calmodulin (Cam)‐Cam kinase II (CamK II) pathway regulates phagosome maturation in macrophages, and this pathway is inhibited by pathogenic microbes. Our hypothesis is that signal transduction events which control phagosome maturation also regulate antigen presentation. Stimulation of primary human DC or the human DC line KG‐1, with particulate antigen, resulted in the activation of CamK II and its localization to the phagosome and plasma membrane. Two mechanistically distinct inhibitors of CamK II significantly reduced DC maturation, as determined by up‐regulation of surface costimulatory and major histocompatibility complex (MHC) class II molecules and secretion of cytokines. Confocal microscopy demonstrated that the CamK II inhibitors blocked the antigen‐induced increase in total cellular MHC class molecules as well as their trafficking to the plasma membrane. Inhibition of CamK II was associated with decreased presentation of particulate and soluble MHC class II‐restricted antigen, with a greater effect on the former. These data support a model in which CamK II regulates critical stages of the maturation and antigen presentation capacity of human DC, particularly in response to stimulation via phagocytosis.