2 mice with repeated administrations of low-dose CYC at longer time intervals (coinciding with the blocked renewal of Tregs) resulted in reduced tumor load and prevention or delay of disease recurrence. The break of immune tolerance against MM tumor cells by prolonged maintenance of transient Treg depletion will be reviewed. 2. Immune Abnormalities in MM Patients The number and function of T cells subsets are aberrant in patients with MM [5, 6]. The CD4 : CD8 ratio is inverted and the helper T-cell type 1 to type 2 (Th1 : Th2) ratio among CD4 cells is abnormal [7]. In addition, the levels of expression of CD28 costimulatory molecules required for T cell activation are downregulated in T cells derived from MM patients [8]. The elevated levels of transforming growth factor (TGF)-β, in addition to the impaired accessory signals from Th cells, contribute to the presence of dysfunctional B cells [9]. Defective natural killer cells (NK) have also been noted in MM patients [10]. Circulating dendritic cells (DCs) from MM patients were shown to be dysfunctional, failing to upregulate costimulatory molecules required for DCs activation, which led to reduced phagocytic activity and antigen presentation [11]. It was reported that the impaired activity of DCs may be linked to the upregulation of Tregs [12]. Interestingly, in vivo expansion of Tregs could be induced in the presence of MM-speciﬁc antigens in association with reduced immune eﬀector functions. Immunologically active compounds that promote tumor-cell survival are produced by myeloma cells. The latter include TGF-β, IL-10, IL-6, vascular endothelial angiogenic growth factor (VEGF), Fas ligand, Mucin 1, cyclooxygenase (Cox)- 2, and related prostanoids and metalloproteins [13]. The increased numbers of Treg and the impairment and mod- ulation of immune functions in myeloma patients suggest that breaking tolerance by chemoimmunotherapy involving transient Treg depletion and recruitment of compatible immune-derived cells could perhaps reduce tumor load and delay or prevent tumor development. 3. CD4+CD25highFoxp3+ Tregs Subpopulations of T cells with suppressive capacities against antitumor activity of the immune system were ﬁrst described in the early 1980s [14]. In the last decade, these subsets of cells were found to be naturally occurring or adoptively induced. They constitute 5–10% of na¨ıve CD4+ T cells in the periphery and are anergic cells with suppressive capabilities. These cells are induced early during tumor development and were shown to contribute to tumor tolerance [15]. These cells were characterized as CD4 cells that express CD25 and the transcription factor Foxp3 (forkhead/winged helix tran- scription factor). The latter is essential for the suppressive phenotype of the cells [16, 17]. Also, Tregs constitutively express CTLA-4, and CTLA-4 deﬁciency in Tregs results in enhanced antitumor immunity [18]. The mechanisms underlying the suppressive eﬀects of Tregs include inhibiting the activity of a variety of immune cells that are tumor Bone Marrow Research speciﬁc such as CD4 and CD8 cells, B cells, NK cells, natural killer T cells (NKT), and DCs [19–21]. Both types of T cells, CD4 and CD8 cells, are considered key components against tumors [22, 23]. Further, NK cells destroy tumor cells that have reduced the expression of MHC class I but still express ligands for activating receptors of NK cells [24]. These cells are prone to inhibition by Tregs, which might lead to the continuation of tumor development and proliferation. 3.1. Tregs and Malignancy. Tregs, naturally occurring and inducible, play a key role in the regulation of antitumor immune response in patients with cancer. It was shown that within the tumor microenvironment the tumor itself induces na¨ıve CD4 cells to convert into antigen-speciﬁc Tregs [15]. Many of the immune cells that reside in the tumor milieu are aﬀected by Tregs, and thus Tregs help tumors to escape detection and elimination by the immune system. It was shown that depletion of Tregs in tumor-bearing mice resulted in the enhancement of antitumor immunity and the reduction of tumor growth [25–29]. In patients with ovarian cancer, the inﬁltration of CD3+ eﬀector T cells was associated with a better prognosis [30], whereas the accumulation of Tregs was suggestive of poor prognosis [2]. These observations were also conﬁrmed in non-small-cell lung cancer [31]. The accumulated data strongly suggested a new therapeutic strategy in the ﬁeld of cancer aimed at eliminating Tregs or disrupting their activity. 3.2. Treg and MM. There is now suﬃcient evidence that CD4+CD25highFoxp3+ Tregs actively impede the antitumor immune response in cancer patients. In vivo peripheral expansion of natural na¨ıve CD4+CD25highFoxp3+ Tregs were observed in large cohorts of 195 MM patients [32, 33]. Increased numbers of naturally occurring Tregs correlated with disease burden (coinciding with paraprotein levels) and increased morbidity. The frequency of double negative T cells (CD3+, CD4−, and CD8− αβTCR+) was reduced in patients with MM. CD4+CD25high T cells from MM patients were shown to coexpress markers associated with Treg phenotype such as CTLA-4, GITR, and CD69. High serum levels of IL-10 and TGF-β were also observed. In both humans and animal models of MM, Tregs have been described as anergic cells, fully functional in early and late stage MM, exerting strong suppression after T-cell receptor stimulation [34, 35]. Contradictory ﬁndings concerning Treg levels and activity in MM patients were published by Prabhala et al. [36], namely, reduced CD4+Foxp3+ T cells as well as Treg dysfunction, indicating that Tregs were unable to suppress anti-CD3+ mediated T-cell proliferation. Whether these tested cells were coexpressing CD25high was not determined in this study. Notably, it seems that diﬀerences in research strategies may account for the contradictory data in the area of MM malignancy. The response of Tregs to tumors is illustrated in mice that lack Tregs and eﬀectively reject tumors [26]. The involvement of Tregs in MM progression was also studied in a unique mouse model of MM (5T2MM) that developed sponta- neously in BM of a very old mouse of the C57BL/KalwRij strain [37, 38]. The tumor designated 5T2MM can be