Indoleamine 2,3-dioxygenase 1 is essential for sustaining durable antibody responses.

Humoral immunity is essential for protection against pathogens, emphasized by the prevention of 2-3 million deaths worldwide annually by childhood immunizations. Long-term protective immunity is dependent on the continual production of neutralizing antibodies by the subset of long-lived plasma cells (LLPCs). LLPCs are not intrinsically long-lived, but require interaction with LLPC niche stromal cells for survival. However, it remains unclear which and how these interactions sustain LLPC survival and long-term humoral immunity. We now have found that the immunosuppressive enzyme indoleamine 2,3- dioxygenase 1 (IDO1) is required to sustain antibody responses and LLPC survival. Activation of IDO1 occurs upon the engagement of CD80/CD86 on the niche dendritic cells by CD28 on LLPC. Kynurenine, the product of IDO1 catabolism, activates the aryl hydrocarbon receptor in LLPC, reinforcing CD28 expression and survival signaling. These findings expand the immune function of IDO1 and uncover a novel pathway for sustaining LLPC survival and humoral immunity.

CD28 Promotes Plasma Cell Survival, Sustained Antibody Responses, and BLIMP-1 Upregulation through Its Distal PYAP Proline Motif.

In health, long-lived plasma cells (LLPC) are essential for durable protective humoral immunity, and, conversely, in disease are a major source of pathogenic Abs in autoimmunity, graft rejection, and allergy. However, the molecular basis for their longevity is largely unknown. We have recently found that CD28 signaling in plasma cells (PC) is essential for sustaining Ab titers, by supporting the survival of LLPC, but not short-lived PC (SLPC). We now find that, unlike SLPC, CD28 activation in LLPC induces prosurvival downstream Vav signaling. Knockin mice with CD28 cytoplasmic tail mutations that abrogate Vav signaling (CD28-AYAA) had significantly fewer LLPC but unaffected SLPC numbers, whereas mice with mutations that abrogate PI3K signaling (CD28-Y170F) were indistinguishable from wild-type controls. This was consistent with the loss of CD28's prosurvival effect in LLPC from CD28-AYAA, but not CD28-Y170F, mice. Furthermore, the CD28 Vav motif in the B lineage was essential for the long-term maintenance of Ag-specific LLPC populations and Ab titers in vivo. Signaling downstream of the CD28 Vav motif induced previously undescribed transcriptional regulation of B lymphocyte-induced maturation protein-1, a key mediator of PC differentiation and maintenance. These findings suggest CD28 signaling in LLPC modulates the central B lymphocyte-induced maturation protein-1 transcriptional nexus involved in long-term survival and function.

CD28-mediated pro-survival signaling induces chemotherapeutic resistance in multiple myeloma.

Chemotherapeutic resistance remains a significant hurdle in the treatment of multiple myeloma (MM) and is significantly mediated by interactions between MM cells and stromal cells of the bone marrow microenvironment. Despite the importance of these interactions, the specific molecules and downstream signaling components involved remain incompletely understood. We have previously shown that the prototypic T-cell costimulatory receptor CD28, which is also expressed on MM cells, is a key mediator of MM survival and apoptotic resistance. Crosslinking CD28 by agonistic antibodies or myeloid dendritic cells (DC; these express the CD28 ligands CD80/CD86) prevents apoptosis caused by chemotherapy or serum withdrawal. We now report that CD28 pro-survival signaling is dependent upon downstream activation of phosphatidyl-inositol 3-kinase/Akt, inactivation of the transcription factor FoxO3a, and decreased expression of the pro-apoptotic molecule Bim. Conversely, blocking the CD28-CD80/CD86 interaction between MM cells and DC in vitro abrogates the DC's ability to protect MM cells against chemotherapy-induced death. Consistent with these observations, in vivo blockade of CD28-CD80/CD86 in the Vk*MYC murine myeloma model sensitizes MM cells to chemotherapy and significantly reduces tumor burden. Taken together, our findings suggest that CD28 is an important mediator of MM survival during stress and can be targeted to overcome chemotherapy resistance.

Novel regulation of CD80/CD86-induced phosphatidylinositol 3-kinase signaling by NOTCH1 protein in interleukin-6 and indoleamine 2,3-dioxygenase production by dendritic cells.

Dendritic cells (DC) play a critical role in modulating antigen-specific immune responses elicited by T cells via engagement of the prototypic T cell costimulatory receptor CD28 by the cognate ligands CD80/CD86, expressed on DC. Although CD28 signaling in T cell activation has been well characterized, it has only recently been shown that CD80/CD86, which have no demonstrated binding domains for signaling proteins in their cytoplasmic tails, nonetheless also transduce signals to the DC. Functionally, CD80/CD86 engagement results in DC production of the pro-inflammatory cytokine IL-6, which is necessary for full T cell activation. However, ligation of CD80/CD86 by CTLA4 also induces DC production of the immunosuppressive enzyme indoleamine 2,3-dioxygenase (IDO), which depletes local pools of the essential amino acid tryptophan, resulting in blockade of T cell activation. Despite the significant role of CD80/CD86 in immunological processes and the seemingly opposing roles they play by producing IL-6 and IDO upon their activation, how CD80/CD86 signal remains poorly understood. We have now found that cross-linking CD80/CD86 in human DC activates the PI3K/AKT pathway. This results in phosphorylation/inactivation of its downstream target, FOXO3A, and alleviates FOXO3A-mediated suppression of IL-6 expression. A second event downstream of AKT phosphorylation is activation of the canonical NF-κB pathway, which induces IL-6 expression. In addition to these downstream pathways, we unexpectedly found that CD80/CD86-induced PI3K signaling is regulated by previously unrecognized cross-talk with NOTCH1 signaling. This cross-talk is facilitated by NOTCH-mediated up-regulation of the expression of prolyl isomerase PIN1, which in turn increases enzyme activity of casein kinase II. Subsequently, phosphatase and tensin homolog (which suppresses PI3K activity) is inactivated via phosphorylation by casein kinase II. This results in full activation of PI3K signaling upon cross-linking CD80/CD86. Similar to IL-6, we have found that CD80/CD86-induced IDO production by DC at late time points is also dependent upon the PI3K → AKT → NF-κB pathway and requires cross-talk with NOTCH signaling. These data further suggest that the same signaling pathways downstream of DC CD80/CD86 cross-linking induce early IL-6 production to enhance T cell activation, followed by later IDO production to self-limit this activation. In addition to characterizing the pathways downstream of CD80/CD86 in IL-6 and IDO production, identification of a novel cross-talk between NOTCH1 and PI3K signaling may provide new insights in other biological processes where PI3K signaling plays a major role.

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.

Mitigating the prevalence and function of myeloid-derived suppressor cells by redirecting myeloid differentiation using a novel immune modulator.

BACKGROUND: Immune suppression is common in neoplasia and a major driver is tumor-induced myeloid dysfunction. Yet, overcoming such myeloid cell defects remains an untapped strategy to reverse suppression and improve host defense. Exposure of bone marrow progenitors to heightened levels of myeloid growth factors in cancer or following certain systemic treatments promote abnormal myelopoiesis characterized by the production of myeloid-derived suppressor cells (MDSCs) and a deficiency in antigen-presenting cell function. We previously showed that a novel immune modulator, termed 'very small size particle' (VSSP), attenuates MDSC function in tumor-bearing mice, which was accompanied by an increase in dendritic cells (DCs) suggesting that VSSP exhibits myeloid differentiating properties. Therefore, here, we addressed two unresolved aspects of the mechanism of action of this unique immunomodulatory agent: (1) does VSSP alter myelopoiesis in the bone marrow to redirect MDSC differentiation toward a monocyte/macrophage or DC fate? and (2) does VSSP mitigate the frequency and suppressive function of human tumor-induced MDSCs? METHODS: To address the first question, we first used a murine model of granulocyte-colony stimulating factor-driven emergency myelopoiesis following chemotherapy-induced myeloablation, which skews myeloid output toward MDSCs, especially the polymorphonuclear (PMN)-MDSC subset. Following VSSP treatment, progenitors and their myeloid progeny were analyzed by immunophenotyping and MDSC function was evaluated by suppression assays. To strengthen rigor, we validated our findings in tumor-bearing mouse models. To address the second question, we conducted a clinical trial in patients with metastatic renal cell carcinoma, wherein 15 patients were treated with VSSP. Endpoints in this study included safety and impact on PMN-MDSC frequency and function. RESULTS: We demonstrated that VSSP diminished PMN-MDSCs by shunting granulocyte-monocyte progenitor differentiation toward monocytes/macrophages and DCs with heightened expression of the myeloid-dependent transcription factors interferon regulatory factor-8 and PU.1. This skewing was at the expense of expansion of granulocytic progenitors and rendered the remaining MDSCs less suppressive. Importantly, these effects were also demonstrated in a clinical setting wherein VSSP monotherapy significantly reduced circulating PMN-MDSCs, and their suppressive function. CONCLUSIONS: Altogether, these data revealed VSSP as a novel regulator of myeloid biology that mitigates MDSCs in cancer patients and reinstates a more normal myeloid phenotype that potentially favors immune activation over immune suppression.

6,7-dihydroxy-3,4-dihydroisoquinoline: a novel inhibitor of nuclear factor-kappaB and in vitro invasion in murine mammary cancer cells.

BACKGROUND: The inhibition of nuclear factor (NF)-kappaB with nontoxic agents is a promising possible treatment approach that may inhibit tumor cell proliferation, counteract the prosurvival pathways that mediate resistance to cytotoxic therapy, and prevent tumor cell metastasis. METHODS: An initial structure-activity relationship study of the NF-kappaB inhibitory activity of acetophenone-type compounds using electrophoretic mobility shift assay and Western blot analysis is presented. An in vitro cell invasion assay using DA3 cells, a murine breast cancer cell line, was conducted to model antimetastatic activity. RESULTS: The carbonyl moiety is found to be the functional group responsible for inhibition of NF-kappaB, and a novel, more effective agent, 6,7-dihydroxy-3,4-dihydroisoquinoline, is postulated and confirmed. The compounds are characterized as active in the inhibition of both the canonical and noncanonical NF-kappaB signaling pathways. Lastly, 6,7-dihydroxy-3,4-dihydroisoquinoline is discovered to inhibit in vitro invasion in DA3 cells. CONCLUSION: 6,7-Dihydroxy-3,4-dihydroisoquionoline and its derivatives are presented as potential prototypes for a novel series of nontoxic antimetastatic agents that can be used in conjunction with current cancer therapeutic techniques.

Regulation of RelB expression during the initiation of dendritic cell differentiation.

The transcription factor RelB is required for proper development and function of dendritic cells (DCs), and its expression is upregulated early during differentiation from a variety of progenitors. We explored this mechanism of upregulation in the KG1 cell line model of a DC progenitor and in the differentiation-resistant KG1a subline. RelB expression is relatively higher in untreated KG1a cells but is upregulated only during differentiation of KG1 by an early enhancement of transcriptional elongation, followed by an increase in transcription initiation. Restoration of protein kinase CbetaII (PKCbetaII) expression in KG1a cells allows them to differentiate into DCs. We show that PKCbetaII also downregulated constitutive expression of NF-kappaB in KG1a-transfected cells and restores the upregulation of RelB during differentiation by increased transcriptional initiation and elongation. The two mechanisms are independent and sensitive to PKC signaling levels. Conversely, RelB upregulation was inhibited in primary human monocytes where PKCbetaII expression was knocked down by small interfering RNA targeting. Altogether, the data show that RelB expression during DC differentiation is controlled by PKCbetaII-mediated regulation of transcriptional initiation and elongation.

Tumor-induced STAT3 signaling in myeloid cells impairs dendritic cell generation by decreasing PKCßII abundance.

A major mechanism by which cancers escape control by the immune system is by blocking the differentiation of myeloid cells into dendritic cells (DCs), immunostimulatory cells that activate antitumor T cells. Tumor-dependent activation of signal transducer and activator of transcription 3 (STAT3) signaling in myeloid progenitor cells is thought to cause this block in their differentiation. In addition, a signaling pathway through protein kinase C ßII (PKCßII) is essential for the differentiation of myeloid cells into DCs. We found in humans and mice that breast cancer cells substantially decreased the abundance of PKCßII in myeloid progenitor cells through a mechanism involving the enhanced activation of STAT3 signaling by soluble, tumor-derived factors (TDFs). STAT3 bound to previously undescribed negative regulatory elements within the promoter of PRKCB, which encodes PKCßII. We also found a previously undescribed counter-regulatory mechanism through which the activity of PKCßII inhibited tumor-dependent STAT3 signaling by decreasing the abundance of cell surface receptors, such as cytokine and growth factor receptors, that are activated by TDFs. Together, these data suggest that a previously unrecognized cross-talk mechanism between the STAT3 and PKCßII signaling pathways provides the molecular basis for the tumor-induced blockade in the differentiation of myeloid cells, and suggest that enhancing PKCßII activity may be a therapeutic strategy to alleviate cancer-mediated suppression of the immune system.

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

Sustained long-term antibody levels are the cornerstone of protective immunity, yet it remains unclear how they are durably maintained. A predominant theory implicates antigen-independent antibody production by a subset of long-lived plasma cells (LLPCs) that survive within bone marrow (BM). Central tenets of this model--that BM LLPCs constitute a subset defined by intrinsic biology distinct from PCs in other tissues and contribute to long-term antibody titers--have not been definitively demonstrated. We now report that long-term humoral immunity depends on the PC-intrinsic function of CD28, which selectively supports the survival of BM LLPC but not splenic short-lived PC (SLPC). LLPC and SLPC both express CD28, but CD28-driven enhanced survival occurred only in the LLPC. In vivo, even in the presence of sufficient T cell help, loss of CD28 or its ligands CD80 and CD86 caused significant loss of the LLPC population, reduction of LLPC half-life from 426 to 63 d, and inability to maintain long-term antibody titers, but there was no effect on SLPC populations. These findings establish the existence of the distinct BM LLPC subset necessary to sustain antibody titers and uncover a central role for CD28 function in the longevity of PCs and humoral immunity.

Tumor-mediated inhibition of dendritic cell differentiation is mediated by down regulation of protein kinase C beta II expression.

Tumor-mediated immune suppression occurs through multiple mechanisms, including dysregulation of dendritic cell differentiation. This block in differentiation results in fewer dendritic cells and an accumulation of immunosuppressive myeloid- derived suppressor cells and is thought to contribute to tumor outgrowth and to act as an impediment to successful anti-cancer immunotherapy. Tumor-mediated myeloid dysregulation is known to be Stat3 dependent; however, the molecular mechanism of this Stat3 signaling remains poorly defined. We have previously shown that PKC betaII is required for dendritic cell differentiation. Here, we describe our finding that tumors mediate both Stat3 activation and PKC betaII down regulation in DC progenitor cells, a process mimicked by the expression of a constitutive active Stat3 mutant. This demonstrates that tumor-mediated myeloid dysregulation may be mediated by Stat3- induced PKC betaII down regulation.

Modulation of dendritic cell differentiation and function by YopJ of Yersinia pestis.

Yersinia pestis evades immune responses in part by injecting into host immune cells several effector proteins called Yersinia outer proteins (Yops) that impair cellular function. This has been best characterized in the innate effector cells, but much less so for cells involved in adaptive immune responses. Dendritic cells (DC) sit at the crossroads between innate and adaptive immunity, and can function to initiate or inhibit adaptive immune responses. Although Y. pestis can target and inactivate DC, the mechanism responsible for this remains unclear. We have found that injection of Y. pestis YopJ into DC progenitors disrupts key signal transduction pathways and interferes with DC differentiation and subsequent function. YopJ injection prevents up-regulation of the NF-kappaB transcription factor Rel B and inhibits MAPK/ERK activation--both having key roles in DC differentiation. Furthermore, YopJ injection prevents costimulatory ligand up-regulation, LPS-induced cytokine expression, and yields differentiated DC with diminished capability to induce T cell proliferation and IFN-gamma induction. By modulating DC function through YopJ-mediated disruption of signaling pathways during progenitor to DC differentiation, Yersinia may interfere with the adaptive responses necessary to clear the infection as well as establish a tolerant immune environment that leads to chronic infection/carrier state in the surviving host.

CD28-mediated regulation of multiple myeloma cell proliferation and survival.

Although interactions with bone marrow stromal cells are essential for multiple myeloma (MM) cell survival, the specific molecular and cellular elements involved are largely unknown, due in large part to the complexity of the bone marrow microenvironment itself. The T-cell costimulatory receptor CD28 is also expressed on normal and malignant plasma cells, and CD28 expression in MM correlates significantly with poor prognosis and disease progression. In contrast to T cells, activation and function of CD28 in myeloma cells is largely undefined. We have found that direct activation of myeloma cell CD28 by anti-CD28 mAb alone induces activation of PI3K and NFkappaB, suppresses MM cell proliferation, and protects against serum starvation and dexamethasone (dex)-induced cell death. Coculture with dendritic cells (DCs) expressing the CD28 ligands CD80 and CD86 also elicits CD28-mediated effects on MM survival and proliferation, and DCs appear to preferentially localize within myeloma infiltrates in primary patient samples. Our findings suggest a previously undescribed myeloma/DC cell-cell interaction involving CD28 that may play an important role in myeloma cell survival within the bone marrow stroma. These data also point to CD28 as a potential therapeutic target in the treatment of MM.

Proteasome inhibitors induce a terminal unfolded protein response in multiple myeloma cells.

Multiple myeloma (MM) is an incurable plasma cell malignancy. The 26S proteasome inhibitor, bortezomib, selectively induces apoptosis in MM cells; however, the nature of its selectivity remains unknown. Here we demonstrate that 5 different MM cell lines display similar patterns of sensitivity to 3 proteasome inhibitors (PIs) but respond differently to specific NF-kappaB inhibition. We further show that PIs initiate the unfolded protein response (UPR), a signaling pathway activated by the accumulation of misfolded proteins within the endoplasmic reticulum (ER). Consistent with reports that prosurvival/physiologic UPR components are required for B-cell differentiation into antibody-secreting cells, we found that MM cells inherently expressed the ER chaperones GRP78/Bip and GRP94/gp96. However, bortezomib rapidly induced components of the proapoptotic/terminal UPR, including PERK, the ER stress-specific eIF-2alpha kinase; ATF4, an ER stress-induced transcription factor; and its proapoptotic target, CHOP/GADD153. Consistent with our hypothesis that PIs induce the accumulation of misfolded ER-processed proteins, we found that the amount of immunoglobulin subunits retained within MM cells correlated with their sensitivity to PIs. These findings suggest that MM cells have a lower threshold for PI-induced UPR induction and ER stress-induced apoptosis because they constitutively express ER stress survival factors to function as secretory cells.

Differentiation of acute and chronic myeloid leukemic blasts into the dendritic cell lineage: analysis of various differentiation-inducing signals.

PURPOSE: Ex vivo differentiation of myeloid leukemic blasts into dendritic cells (DCs) holds significant promise for use as cellular vaccines, as they may present a constellation of endogenously expressed known and unknown leukemia antigens to the immune system. Although variety of stimuli can drive leukemia --> DC differentiation in vitro, these blast-derived DCs typically have aberrant characteristics compared with DCs generated from normal progenitors by the same stimuli. It is not clear whether this is due to underlying leukemogenic mechanisms (e.g., specific oncogenes), genetic defects, stage of maturation arrest, defects in cytokine receptor expression or signal transduction pathways, or whether different stimuli themselves induce qualitatively dissimilar DC differentiation. METHODS: To assess what factors may contribute to aberrant leukemic blast --> DC differentiation, we have examined how the same leukemic blasts (AML and CML) respond to different DC differentiation signals--including extracellular (the cytokine combination GM-CSF + TNF-alpha + IL-4) and intracellular (the protein kinase C agonist PMA, the calcium ionophore A23187, and the combination of PMA plus A23187) stimuli. RESULTS: We have found that the same leukemic blasts will develop qualitatively different sets of DC characteristics in response to differing stimuli, although no stimuli consistently induced all of the characteristic DC features. There were no clear differences in the responses relative to specific oncogene expression or stage of maturation arrest (AML vs CML). Signal transduction agonists that bypassed membrane receptors/proximal signaling (in particular, the combination of PMA and A23187) consistently induced the greatest capability to activate T cells. Interestingly, this ability did not clearly correlate with expression of MHC/costimulatory ligands. CONCLUSIONS: Our findings suggest that signal transduction may play an important role in the aberrant DC differentiation of leukemic blasts, and demonstrate that direct activation of PKC together with intracellular calcium signaling may be an effective method for generating immunostimulatory leukemia-derived DCs.

Protein kinase C betaII plays an essential role in dendritic cell differentiation and autoregulates its own expression.

Dendritic cells (DC) arise from a diverse group of hematopoietic progenitors and have marked phenotypic and functional heterogeneity. The signal transduction pathways that regulate the ability of progenitors to undergo DC differentiation, as well as the specific characteristics of the resulting DC, are only beginning to be characterized. We have found previously that activation of protein kinase C (PKC) by cytokines or phorbol esters drives normal human CD34(+) hematopoietic progenitors and myeloid leukemic blasts (KG1, K562 cell lines, and primary patient blasts) to differentiate into DC. We now report that PKC activation is also required for cytokine-driven DC differentiation from monocytes. Of the cPKC isoforms, only PKC-betaII was consistently activated by DC differentiation-inducing stimuli in normal and leukemic progenitors. Transfection of PKC-betaII into the differentiation-resistant KG1a subline restored the ability to undergo DC differentiation in a signal strength-dependent fashion as follows: 1) by development of characteristic morphology; 2) the up-regulation of DC surface markers; 3) the induction of expression of the NFkappaB family member Rel B; and 4) the potent ability to stimulate allo-T cells. Most unexpectedly, the restoration of PKC-betaII signaling in KG1a was not directly due to overexpression of the transfected classical PKC (alpha, betaII, or gamma) but rather through induction of endogenous PKC-beta gene expression by the transfected classical PKC. The mechanism of this positive autoregulation involves up-regulation of PKC-beta promoter activity by constitutive PKC signaling. These findings indicate that the regulation of PKC-betaII expression and signaling play critical roles in mediating progenitor to DC differentiation.

Induced dendritic cell differentiation of chronic myeloid leukemia blasts is associated with down-regulation of BCR-ABL.

Although differentiation of leukemic blasts to dendritic cells (DC) has promise in vaccine strategies, the mechanisms underlying this differentiation and the differences between leukemia and normal progenitor-derived DC are largely undescribed. In the case of chronic myeloid leukemia (CML), understanding the relationship between the induction of DC differentiation and the expression of the BCR-ABL oncogene has direct relevance to CML biology as well as the development of new therapeutic approaches. We now report that direct activation of protein kinase C (PKC) by the phorbol ester PMA in the BCR-ABL(+) CML cell line K562 and primary CML blasts induced nonterminal differentiation into cells with typical DC morphology (cytoplasmic dendrites), characteristic surface markers (MHC class I, MHC class II, CD86, CD40), chemokine and transcription factor expression, and ability to stimulate T cell proliferation (equivalent to normal monocyte-derived DC). PKC-induced differentiation was associated with down-regulation of BCR-ABL mRNA expression, protein levels, and kinase activity. This down-regulation appeared to be signaled through the mitogen-activated protein kinase pathway. Therefore, PKC-driven differentiation of CML blasts into DC-like cells suggests a potentially novel strategy to down-regulate BCR-ABL activity, yet raises the possibility that CML-derived DC vaccines will be less effective in presenting leukemia-specific Ags.