Alterations in macrophages and monocytes from tumor-bearing mice: evidence of local and systemic immune impairment

IL-4-primed human umbilical cord mesenchymal stem cells-derived extracellular vesicles facilitate recovery in spinal cord injury via the miR-21-5p/PDCD4-mediated shifting of macrophage M1/M2 polarization

Spinal cord injury (SCI) represents a significant neurological disorder that profoundly impacts human life. Transplantation of extracellular vesicles (EVs) from human umbilical cord mesenchymal stem cells (hUC-MSCs) has emerged as a promising therapeutic strategy. microRNA (miRNA) containing EVs serve as crucial mediators of intercellular communication, playing vital roles in physiological and pathological processes. Research indicates that EVs from hUC-MSCs could attenuate inflammation and facilitate recovery from SCI. Nevertheless, their application in clinical treatment necessitates further investigation. We are actively pursuing an effective approach to modulate the intensity of the inflammatory response, thereby addressing secondary SCI. Initially, we activated hUC-MSCs with interleukin-4 (IL-4) and subsequently harvested their EVs. We investigated the influences of A-hUC-MSCs-EVs compared to routinely acquired EVs on macrophage polarization phenotypes both in vitro and in vivo. Our results show that EVs originating from A-hUC-MSCs are more effective at promoting macrophage polarization from the M1 phenotype to the M2 phenotype than those derived from hUC-MSCs. Notably, we found that A-hUC-MSCs-derived EVs had a superior impact on motor function recovery in mice with SCI. Importantly, we observed that IL-4 activation significantly upregulated the expression of miR-21-5p within these EVs. More specifically, our data demonstrate that A-hUC-MSCs-EVs depend on miR-21-5p to inhibit the effects of PDCD4 on macrophage polarization. This mechanism regulates inflammatory responses while simultaneously reducing apoptosis. In summary, EVs derived from IL-4 primed hUC-MSCs are enriched with miR-21-5p, which exerts a pivotal influence in shifting macrophage polarization, alleviating inflammatory responses following SCI, and facilitating recovery.

Focus on Pancreatic Cancer Microenvironment

Pancreatic ductal adenocarcinoma remains one of the most lethal solid tumors due to its local aggressiveness and metastatic potential, with a 5-year survival rate of only 13%. A robust connection between pancreatic cancer microenvironment and tumor progression exists, as well as resistance to current anticancer treatments. Pancreatic cancer has a complex tumor microenvironment, characterized by an intricate crosstalk between cancer cells, cancer-associated fibroblasts and immune cells. The complex composition of the tumor microenvironment is also reflected in the diversity of its acellular components, such as the extracellular matrix, cytokines, growth factors and secreted ligands involved in signaling pathways. Desmoplasia, the hallmark of the pancreatic cancer microenvironment, contributes by creating a dense and hypoxic environment that promotes further tumorigenesis, provides innate systemic resistance and suppresses anti-tumor immune invasion. We discuss the complex crosstalk among tumor microenvironment components and explore therapeutic strategies and opportunities in pancreatic cancer research. Better understanding of the tumor microenvironment and its influence on pancreatic cancer progression could lead to potential novel therapeutic options, such as integration of immunotherapy and cytokine-targeted treatments.

Myeloid Immune Cells CARrying a New Weapon Against Cancer

Chimeric antigen receptor (CAR) engineering for T cells and natural killer cells (NK) are now under clinical evaluation for the treatment of hematologic cancers. Although encouraging clinical results have been reported for hematologic diseases, pre-clinical studies in solid tumors have failed to prove the same effectiveness. Thus, there is a growing interest of the scientific community to find other immune cell candidate to express CAR for the treatment of solid tumors and other diseases. Mononuclear phagocytes may be the most adapted group of cells with potential to overcome the dense barrier imposed by solid tumors. In addition, intrinsic features of these cells, such as migration, phagocytic capability, release of soluble factors and adaptive immunity activation, could be further explored along with gene therapy approaches. Here, we discuss the elements that constitute the tumor microenvironment, the features and advantages of these cell subtypes and the latest studies using CAR-myeloid immune cells in solid tumor models.

B cell depletion inhibits fibrosis via suppressing pro-fibrotic macrophage differentiation in a mouse model of systemic sclerosis

OBJECTIVE

We investigated the effect of B cell depletion on fibrosis in systemic sclerosis (SSc) and its mechanism of action.

METHODS

Bleomycin-induced SSc (BLM-SSc) model mice were treated with anti-CD20 antibody, and skin and lung fibrosis was evaluated histopathologically. T cells and macrophages were co-cultured with B cells, and the effect of B cells on their differentiation was assessed by flow cytometry. We also co-cultured B cells and monocytes from SSc patients and analyzed the correlation between fibrosis and pro-fibrotic macrophage induction by B cells.

RESULTS

B cell depletion inhibited fibrosis in BLM-SSc mice. B cells from BLM-SSc mice increased pro-inflammatory cytokine producing T cells in co-culture. In BLM-SSc mice, B cell depletion before BLM treatment (pre-depletion) inhibited fibrosis more strongly than B cell depletion after BLM treatment (post-depletion). However, the frequencies of pro-inflammatory T cells were lower in the post-depletion group than in the pre-depletion group. This discrepancy suggests that the effect of B cell depletion on fibrosis cannot be explained by its effect on T cell differentiation. On the other hand, pro-fibrotic macrophages were markedly decreased in the pre-depletion group than in the post-depletion group. Furthermore, B cells from BLM-SSc mice increased pro-fibrotic macrophage differentiation in co-culture. In SSc patients, the extent of pro-fibrotic macrophage induction by B cells correlated with the severity of fibrosis.

CONCLUSION

These findings suggest that B cell depletion inhibits tissue fibrosis via suppressing pro-fibrotic macrophage differentiation in BLM-SSc mice, providing a new rationale for B cell depletion therapy in SSc.

Exosomal miR-21 secreted by IL-1β-primed-mesenchymal stem cells induces macrophage M2 polarization and ameliorates sepsis

Sepsis occurs due to a damaging host response to infection and is the chief cause of death in most intensive care units. Mesenchymal stem cells (MSCs) exhibit immunomodulatory properties and can modulate key cells of the innate and adaptive immune systems through various effector mechanisms, such as exosomes. Exosomes and their microRNA (miRNA or miR) cargo including miR-21 can initiate profound phenotypic changes in the tumor microenvironment due to their intercellular communication transmitting the pleiotropic messages between different cell types, tissues, and body fluids. Here, we aimed to characterize the effect of miR-21 delivered from MSC-derived exosomes on the polarization of macrophages in a mouse sepsis model. First, we isolated exosomes from interleukin-1β (IL-1β)-pretreated murine MSCs (βMSCs) and injected them into cecal ligation and puncture (CLP) septic models. We found that βMSCs-derived exosomes could more effectively induce M2-like polarization of macrophages in vitro and in vivo. Administration of βMSCs-derived exosomes attenuated the symptoms in septic mice more effectively and increased their survival rate as compared to exosomes released by naïve MSCs. Importantly, we found that miR-21 was abundantly upregulated in MSCs upon IL-1β stimulation and packaged into exosomes. This exosomal miR-21 was transferred to macrophages, leading to M2 polarization in vitro and in vivo. The therapeutic efficacy of βMSC-derived exosomes was partially lost upon miR-21 inhibition by its specific inhibitors. More specifically, we demonstrated βMSCs-derived exosomes inhibited the effects of PDCD4, the target gene of miR-21, on macrophage polarization and sepsis. In conclusion, exosomal miR-21 emerged as a key mediator of IL-1β pretreatment induced immunomodulatory properties of MSCs. The study indicated a novel basis for therapeutic application of MSCs in sepsis.

CD163+ tumor-associated macrophage accumulation in breast cancer patients reflects both local differentiation signals and systemic skewing of monocytes

Objectives

The accumulation of tumor‐associated macrophages (TAMs) is correlated with poor clinical outcome, but the mechanisms governing their differentiation from circulating monocytes remain unclear in humans.

Methods

Using multicolor flow cytometry, we evaluated TAMs phenotype in 93 breast cancer (BC) patients. Furthermore, monocytes from healthy donors were cultured in the presence of supernatants from dilacerated primary tumors to investigate their differentiation into macrophages (MΦ) in vitro. Additionally, we used transcriptomic analysis to evaluate BC patients’ blood monocytes profiles.

Results

We observed that high intra‐tumor CD163‐expressing TAM density is predictive of reduced survival in BC patients. In vitro, M‐CSF, TGF‐β and VEGF from primary tumor supernatants skewed the differentiation of healthy donor blood monocytes towards CD163^highCD86^lowIL‐10^high M2‐like MΦ that strongly suppressed CD4⁺ T‐cell expansion via PD‐L1 and IL‐10. In addition, blood monocytes from about 40% of BC patients displayed an altered response to in vitro stimulation, being refractory to type‐1 MΦ (M1‐MΦ) differentiation and secreting higher amounts of immunosuppressive, metastatic‐related and angiogenic cytokines. Aside from showing that monocyte transcriptome is significantly altered by the presence of BC, we also demonstrated an overall metabolic de‐activation in refractory monocytes of BC patients. In contrast, monocytes from sensitive BC patients undergoing normal M1‐MΦ differentiation showed up‐regulation of IFN‐response genes and had no signs of metabolic alteration.

Conclusion

Altogether, our results suggest that systemic factors skew BC patient blood monocytes towards a pro‐metastatic profile, resulting in the accumulation of further polarised CD163^high TAMs resembling type‐2 MΦ (M2‐MΦ) in the local BC microenvironment. These data indicate that monitoring circulating monocytes in BC patients may provide an indication of early systemic alterations induced by cancer and, thus, be instrumental in the development of improved personalised immunotherapeutic interventions.

Tumor microenvironment participates in metastasis of pancreatic cancer

Pancreatic cancer is a deadly disease with high mortality due to difficulties in its early diagnosis and metastasis. The tumor microenvironment induced by interactions between pancreatic epithelial/cancer cells and stromal cells is critical for pancreatic cancer progression and has been implicated in the failure of chemotherapy, radiation therapy and immunotherapy. Microenvironment formation requires interactions between pancreatic cancer cells and stromal cells. Components of the pancreatic cancer microenvironment that contribute to desmoplasia and immunosuppression are associated with poor patient prognosis. These components can facilitate desmoplasia and immunosuppression in primary and metastatic sites or can promote metastasis by stimulating angiogenesis/lymphangiogenesis, epithelial-mesenchymal transition, invasion/migration, and pre-metastatic niche formation. Some molecules participate in both microenvironment formation and metastasis. In this review, we focus on the mechanisms of pancreatic cancer microenvironment formation and discuss how the pancreatic cancer microenvironment participates in metastasis, representing a potential target for combination therapy to enhance overall survival.

Immunomodulatory Role of Stem Cells from Human Exfoliated Deciduous Teeth on Periodontal Regeneration

Periodontitis is initiated by the infection of periodontal bacteria and subsequent tissue inflammation due to immunoreaction, eventually leading to periodontal apparatus loss. Stem cells from human exfoliated deciduous teeth (SHEDs) have exhibited beneficial characteristics in dental tissue regeneration. However, the immunomodulatory functions of SHEDs have not been elucidated in the context of periodontitis treatment. In this study, we investigated the potential immunomodulatory effects of SHEDs on experimental periodontitis and demonstrated that multidose delivery of SHEDs led to periodontal tissue regeneration. SHEDs and monocytes/macrophages were cocultured in transwell systems and SHEDs were found to be capable of promoting monocyte/macrophage conversion to CD206⁺ M2-like phenotype. Bioluminescence imaging (BLI) was employed to assess the survival and distribution of SHEDs after delivery in periodontal tissues in an induced periodontitis model, and BLI revealed that SHEDs survived for ∼7 days in periodontal tissues with little tissue diffusion. Then, multidose SHED delivery was applied to treat periodontitis at 7-day intervals. Results showed that mutidose SHEDs altered the cytokine expression profile in gingival crevicular fluid, reduced gum bleeding, increased new attachment of periodontal ligament, and decreased osteoclast differentiation. Micro-computed tomography analysis showed SHED administration significantly increased periodontal regeneration and alveolar bone volume, and decreased distance of cementoenamel junction to alveolar bone crest. Furthermore, an increase in the number of CD206⁺ M2 macrophages was observed in periodontal tissues following the delivery of SHEDs, which aligned well with the promoted conversion to CD206⁺ M2-like cells from monocytes/macrophages in vitro after stimulation by SHEDs. This study demonstrated in a rat periodontitis model that local delivery of SHEDs attributed to the induction of M2 macrophage polarization, reduction of periodontal tissue inflammation, and enhancement of periodontal regeneration.

Targeted therapy for soft tissue sarcomas in adolescents and young adults

Soft tissue sarcomas (STSs) are a heterogeneous group of tumors originating from the mesenchyme. Even though they affect individuals in all age groups, the prevalence of subtypes of STSs changes significantly from childhood through adolescence into adulthood. The mainstay of therapy is surgery, with or without the addition of chemotherapy and/or radiation therapy. These treatment modalities are associated, in many cases, with significant morbidity and, given the heterogeneity of tumor histologies encompassed by the term "STS", have not uniformly improved outcomes. Moreover, some subgroups of STSs appear to be more, and others less, responsive to conventional chemotherapy agents. Over the last two decades, our understanding of the biology of STSs is slowly increasing, allowing for the development of more targeted therapies. We review the new treatment modalities that have been tested on patients with STSs, with a special focus on adolescents and young adults, a group of patients that is often underrepresented in clinical trials and has not received the dedicated attention it deserves, given the significant differences in biology and treatment response in comparison to children and adults.

Myeloid-derived suppressor cells and their role in pancreatic cancer

Pancreatic cancer is a devastating disease and ranks as the third most common cause of cancer-related death. Like many cancers, there has been increased interest in the role of the immune system in the progression and development of pancreatic cancer. In particular, immunosuppression within the tumor microenvironment (TME) is thought to impair the host's antitumor response. In this article, we review myeloid-derived suppressor cells and their contribution to this immunosuppression within the pancreatic TME.

Breast cancers from black women exhibit higher numbers of immunosuppressive macrophages with proliferative activity and of crown-like structures associated with lower survival compared to non-black Latinas and Caucasians

Racial disparities in breast cancer incidence and outcome are a major health care challenge. Patients in the black race group more likely present with an early onset and more aggressive disease. The occurrence of high numbers of macrophages is associated with tumor progression and poor prognosis in solid malignancies. Macrophages are observed in adipose tissues surrounding dead adipocytes in "crown-like structures" (CLS). Here we investigated whether the numbers of CD163+ tumor-associated macrophages (TAMs) and/or CD163+ CLS are associated with patient survival and whether there are significant differences across blacks, non-black Latinas, and Caucasians. Our findings confirm that race is statistically significantly associated with the numbers of TAMs and CLS in breast cancer, and demonstrate that the highest numbers of CD163+ TAM/CLS are found in black breast cancer patients. Our results reveal that the density of CD206 (M2) macrophages is a significant predictor of progression-free survival univariately and is also significant after adjusting for race and for HER2, respectively. We examined whether the high numbers of TAMs detected in tumors from black women were associated with macrophage proliferation, using the Ki-67 nuclear proliferation marker. Our results reveal that TAMs actively divide when in contact with tumor cells. There is a higher ratio of proliferating macrophages in tumors from black patients. These findings suggest that interventions based on targeting TAMs may not only benefit breast cancer patients in general but also serve as an approach to remedy racial disparity resulting in better prognosis patients from minority racial groups.

Role of tumor associated macrophages in regulating pancreatic cancer progression

Pancreatic cancer has an overall 5-year survival rate of less than 5%. Unfortunately, patient survival has not substantially improved in the last couple of decades despite advances in treatment modalities that have been successful in other cancer types. The poor response of pancreatic cancer to therapy is a major obstacle faced by clinicians. Increasing attention is being paid to how tumor cells and non-tumor cells influence each other in the pancreatic tumor microenvironment. Tumor-associated macrophages (TAMs) are a highlight in this field because of their vast presence in the tumor microenvironment. TAMs promote angiogenesis, metastasis, and suppress the anti-tumor immune response. Here we review the current understanding of the role of TAMs in regulating the progression of pancreatic cancer.

BMP-7 Treatment Increases M2 Macrophage Differentiation and Reduces Inflammation and Plaque Formation in Apo E-/- Mice

Inflammation plays a fundamental role in the inception and development of atherosclerosis (ATH). Mechanisms of inflammation include the infiltration of monocytes into the injured area and subsequent differentiation into either pro-inflammatory M1 macrophages or anti-inflammatory M2 macrophages. We have previously published data suggesting bone morphogenetic protein-7 (BMP-7) enhances M2 macrophage differentiation and anti-inflammatory cytokine secretion in vitro. In this regard, we hypothesized BMP-7 would inhibit plaque formation in an animal model of ATH through monocytic plasticity mediation. ATH was generated in male and female Apo E(-/-) mice via partial left carotid artery (PLCA) ligation and mice were divided into 3 groups: Sham, PLCA, and PLCA+BMP-7 (200 ug/kg; i.v.). Our data suggest that BMP-7 inhibits plaque formation and increases arterial systolic velocity. Furthermore, we report inhibition of monocyte infiltration and a decrease in associated pro-inflammatory cytokines (MCP-1, TNF-α, and IL-6) in the PLCA+BMP-7 mice. In contrast, our data suggest a significant (p<0.05) increase in M2 macrophage populations with consequential enhanced anti-inflammatory cytokine (IL-1RA, IL-10, and Arginase 1) expression following BMP-7 treatment. We have also observed that mechanisms promoting monocyte into M2 macrophage differentiation by BMP-7 involve the upregulation and activation of the BMP-7 receptor (BMP-7RII). In conclusion, we report that BMP-7 has the potential to mediate cellular plasticity and mitigate the inflammatory immune response, which results in decreased plaque formation and improved blood velocity.

Subverting Subversion: A Review on the Breast Cancer Microenvironment and Therapeutic Opportunities

This review combines the recent research on the subject of tumor immunology and methods of correcting the immune system's reaction to the tumor microenvironment while impeding the survival and growth of tumor cells, with a focus on breast cancer. Induction of hypoxia-inducible genes in the microenvironment leads to lowering of its pH. This impedes the adaptive immune response and acts to recruit cells of the immune system, which suppress the immune response. Regulatory T-cells (Tregs), myeloid-derived suppressor cells (MDSCs), and their derivatives coordinate an anti-autoimmunity response and a healing response in concert with tumor-secreted cytokines, enzymes, and antigens. Together, they suppress a proper immune reaction to tumor cells and promote cellular reproduction (Fig. 1). In addition, the hypoxia-inducible response and components of the tumor microenvironment such as cancer-associated fibroblasts (CAFs) also create an ideal environment for tumor growth and metastasis via neoangiogenesis and increased motility. Broad-spectrum chemotherapy drugs are problematic as breast cancer cells develop resistance through selective loss of a novel target and downregulation of apoptotic factors. A better understanding of the tumor microenvironment offers new therapeutic opportunities to rescue the immune response, inhibit cancer cell growth pathways, and subvert the tumor microenvironment with little toxicity and side effects.

A transcriptional perspective on human macrophage biology

Macrophages are a major cell type in tissue homeostasis and contribute to both pathology and resolution in all acute and chronic inflammatory diseases ranging from infections, cancer, obesity, atherosclerosis, autoimmune disorders to neurodegenerative diseases such as Alzheimer's disease. The cellular and functional diversity of macrophages depends upon tightly regulated transcription. The innate immune system is under profound evolutionary selection. There is increasing recognition that human macrophage biology differs very significantly from that of commonly studied animal models, which therefore can have a limited predictive value. Here we report on the newest findings on transcriptional control of macrophage activation, and how we envision integrating studies on transcriptional and epigenetic regulation, and more classical approaches in murine models. Moreover, we provide new insights into how we can learn about transcriptional regulation in the human system from larger efforts such as the FANTOM (Functional Annotation of the Mammalian Genome) consortium.

Paracrine Interactions between Adipocytes and Tumor Cells Recruit and Modify Macrophages to the Mammary Tumor Microenvironment: The Role of Obesity and Inflammation in Breast Adipose Tissue

The relationship between obesity and breast cancer (BC) has focused on serum factors. However, the mammary gland contains adipose tissue (AT) which may enable the crosstalk between adipocytes and tumor cells contributing to tumor macrophage recruitment. We hypothesize that the breast AT (bAT) is inflamed in obese females and plays a major role in breast cancer development. The effects of this interplay on macrophage chemotaxis were examined in vitro, using co-cultures of mouse macrophages, mammary tumor cells and adipocytes. Macrophages were exposed to the adipocyte and tumor paracrine factors leptin, CCL2 and lauric acid (alone or in combinations). In cell supernatants Luminex identified additional molecules with chemotactic and other pro-tumor functions. Focus on the adipokine leptin, which has been shown to have a central role in breast cancer pathogenesis, indicated it modulates macrophage phenotypes and functions. In vivo experiments demonstrate that mammary tumors from obese mice are larger and that bAT from obese tumor-bearers contains higher numbers of macrophages/CLS and hypertrophic adipocytes than bAT from lean tumor-bearers, thus confirming it is more inflamed. Also, bAT distal from the tumor is more inflamed in obese than in lean mice. Our results reveal that bAT plays a role in breast cancer development in obesity.

Phospholipid makeup of the breast adipose tissue is impacted by obesity and mammary cancer in the mouse: Results of a pilot study

Obesity, an established risk factor for breast cancer (BC), is associated with systemic inflammation. The breast contains adipose tissue (bAT), yet whether it plays a role in BC progression in obese females is being intensively studied. There is scarce knowledge on the lipid composition of bAT in health and disease. The purpose of this pilot study was: 1) to determine whether obesity and BC are associated with inflammatory changes in bAT 2) to analyze for the first time the lipid profile of bAT in obese and lean mammary tumor-bearing and normal mice. Syngeneic E0771 mammary tumor cells were implanted into the mammary fat pad of lean and diet-induced obese C57BL/6 mice. BATs were analyzed four weeks after tumor cell inoculation by immunohistochemistry and mass spectrometry. Phospholipids were identified and subjected to ratiometric quantification using a TSQ Quantum Access Max triple quadrupole mass spectrometer utilizing precursor ion scan or neutral ion loss scan employing appropriate class specific lipid standards in a two step quantification process. Four main classes of phospholipids were analyzed: phosphatidylcholines phosphatidylserines, phosphatidylethanolamines and phosphatidylinositols. Our results showed that bAT in obese (normal and tumor-bearing) mice contained hypertrophic adipocytes compared with their corresponding samples in lean mice; higher numbers of macrophages and crown-like structures were observed in obese tumor bearers compared to obese normal mice. BAT from normal obese mice revealed higher concentrations of phosphatidylethanolamines. Furthermore, bAT from tumor-bearing mice expressed higher phosphatidylcholines than that from non-tumor bearing mice, suggesting the presence of the tumor is associated with phosphatidylcholines. Conversion of phosphatidylethanolamines to phosphatidylcholines will be investigated in E0771 cells. Additional studies are projected to investigate macrophage activation by these specific classes of phospholipids. Occurrence of triglycerides and free fatty acids will be examined in bAT and similar lipidomic analyses will be carried out visceral adipose tissue, highly inflamed in obesity.

Regulation of Notch 1 signaling in THP-1 cells enhances M2 macrophage differentiation

Macrophage polarization is emerging as an important area of research for the development of novel therapeutics to treat inflammatory diseases. Within the current study, the role of Notch1R in macrophage differentiation was investigated as well as downstream effects in THP-1 monocytes cultured in "inflammation mimicry" media. Interference of Notch signaling was achieved using either the pharmaceutical inhibitor DAPT or Notch1R small interfering RNA (siRNA), and Notch1R expression, macrophage phenotypes, and anti- and proinflammatory cytokine expression were evaluated. Data presented show that Notch1R expression on M1 macrophages as well as M1 macrophage differentiation is significantly elevated during cellular stress (P < 0.05). However, under identical culture conditions, interference to Notch signaling via Notch1R inhibition mitigated these results as well as promoted M2 macrophage differentiation. Moreover, when subjected to cellular stress, macrophage secretion of proinflammatory cytokines was significantly heightened (P < 0.05). Importantly, Notch1R inhibition not only diminished proinflammatory cytokine secretion but also enhanced anti-inflammatory protein release (P < 0.05). Our data suggest that Notch1R plays a pivotal role in M1 macrophage differentiation and heightened inflammatory responses. Therefore, we conclude that inhibition of Notch1R and subsequent downstream signaling enhances monocyte to M2 polarized macrophage outcomes and promotes anti-inflammatory mediation during cellular stress.

An essential role for the immune system in the mechanism of tumor regression following targeted oncogene inactivation

Tumors are genetically complex and can have a multitude of mutations. Consequently, it is surprising that the suppression of a single oncogene can result in rapid and sustained tumor regression, illustrating the concept that cancers are often "oncogene addicted." The mechanism of oncogene addiction has been presumed to be largely cell autonomous as a consequence of the restoration of normal physiological programs that induce proliferative arrest, apoptosis, differentiation, and/or cellular senescence. Interestingly, it has recently become apparent that upon oncogene inactivation, the immune response is critical in mediating the phenotypic consequences of oncogene addiction. In particular, CD4(+) T cells have been suggested to be essential to the remodeling of the tumor microenvironment, including the shutdown of host angiogenesis and the induction of cellular senescence in the tumor. However, adaptive and innate immune cells are likely involved. Thus, the effectors of the immune system are involved not only in tumor initiation, tumor progression, and immunosurveillance, but also in the mechanism of tumor regression upon targeted oncogene inactivation. Hence, oncogene inactivation may be an effective therapeutic approach because it both reverses the neoplastic state within a cancer cell and reactivates the host immune response that remodels the tumor microenvironment.