**5. Discriminating between DCs and macrophages**

It has become clear that DCs, especially DCs from myeloid origin, are very heterogeneous, representing several subtypes with a common origin, but different anatomical locations (lymphoid organs vs. non-lymphoid organs), function and phenotype. Moreover, there is al‐ so a very close relationship between myeloid DCs and macrophages (figure 5).

tion, the number of DC and macrophage subpopulations that can be defined is an exponen‐ tial function of the number of markers that has been examined [59]. Moreover, since each gene/protein has its own intrinsic expression level, the heterogeneity is really unlimited [60]. CD11c, a commonly used DC marker, was already known to be expressed by most tissue macrophages before the use of CD11c-reporter transgenes as markers of DCs, and of CD11c-DTR mice to 'selectively' deplete them [59,61]. Other markers that have been used to track macrophages and DCs in mice include F4/80, CD11b and MHC II, but they have also turned out to be nonspecific [57]. Too little attention has been paid to the expression of antimicrobi‐ al effector molecules, such as lysozyme, which is highly secreted by monocytes and macro‐ phages, but only weakly expressed, if at all, by DCs [62]. Part of the confusion may also result from the flexibility and plasticity of macrophages and from the presence of resident

Dendritic Cells in Atherogenesis: From Immune Shapers to Therapeutic Targets

http://dx.doi.org/10.5772/52900

65

The confusion could be possibly resolved if the appropriate reflections are considered [57]. For example, the correctness of CD11c to identify DCs depends on the anatomical site in question. In the spleen and lymph nodes, mononuclear phagocytes with high expression levels of CD11c – though not those with low or intermediate CD11c – appear to be DCs rath‐ er than macrophages. Accumulating evidence confirms that spleen and lymph node DCs are functionally different from macrophages, do not originate from differentiating monocytes, and share fewer characteristics with monocytes than macrophages [64,65,66]. However, in the lung, high levels of CD11c are expressed on macrophages [67,68], and there are many other anatomical locations apart from the lymphoid organs where macrophages are CD11cpositive. It has been proposed many times that the same set of markers that allows us to dis‐ criminate between DCs and macrophages in lymphoid organs, can also be used in non-

Recent *in vivo* experiments in mice have increased our understanding of the development and functions of DC and macrophage subsets [69,70,71]. However, despite this progress in mice, corresponding human subsets are yet to be characterized. Until now, there is no mor‐ phologic or protein marker of macrophages or DCs which is unambiguous. Moreover, a sin‐ gle set of markers cannot be assumed to apply to all stages of cell differentiation and activation. In conclusion, there is insufficient knowledge to make definitive claims about

If the distinction between DCs and macrophages cannot be made based on morphological features, can it be based on function? Several criteria to define DCs include the property of DCs to localize in the T cell zone of lymphoid organs where they can stimulate T cells, as well as their ability to migrate and carry antigen [72,73]. In contrast, macrophages are best defined by their phagocytic activity and are generally considered as tissue-resident cells. However, recent studies show that macrophages can also migrate and that Langerhans cells (i.e. DCs from the skin and mucosa that carry large Birbeck granules) are not important for T cell priming [74]. In addition, some macrophage subtypes, such as microglia, show only poor phagocytic capacity [57]. Taken together, there is no good functional criterion to define macrophages and monocyte-derived DCs (figure 5), since they represent not just two differ‐

lymphoid organs, but it has become clear that this assumption is not correct.

any marker combination, particularly in non-lymphoid compartments.

and migratory activated DCs in the same organ [63].

The distinction of the differences between macrophages and the heterogeneous family of DCs is notoriously difficult and complicated by the plasticity of both cell types [55]. Mono‐ cytes that exit the blood and enter tissues under inflammatory conditions can differentiate to macrophages, but also to DCs that share several phenotypic features and functions, making it difficult to unambiguously define macrophages and DCs as individual entities [56]. In ad‐ dition, resting peripheral monocytes, obtained from mouse peritoneal cavity lavage, repre‐ sent an immature population, capable of further differentiation along either the dendritic or the macrophage pathway, depending on the type of stimuli (cytokines, growth factors) they receive [10]. Furthermore, many DC subsets are not clearly defined and it is absolutely nec‐ essary to bear in mind that different groups use different methods to identify and character‐ ize DCs [57]. Often, the starting populations are preselected based on randomly defined expression levels of markers that were believed to be specific for either DCs or macrophag‐ es, but are in fact expressed by both [58] (figure 5).


**Figure 5.** Functional characteristics and surface markers of DCs and macrophages. Increasing evidence demonstrates an enormous overlap between what is considered a 'macrophage' and a 'DC'. Abbreviations: MHC II, major histocompatibil‐ *ity complex class II; BDCA-1, blood dendritic-cell antigen-1; DC-SIGN, dendritic cell-specific ICAM-3-grabbing non-integrin.*

Consequently, confusion in distinguishing between macrophages and DCs has been – at least in part – caused by the use of nonspecific cell surface markers, such as CD11c. In addi‐

tion, the number of DC and macrophage subpopulations that can be defined is an exponen‐ tial function of the number of markers that has been examined [59]. Moreover, since each gene/protein has its own intrinsic expression level, the heterogeneity is really unlimited [60]. CD11c, a commonly used DC marker, was already known to be expressed by most tissue macrophages before the use of CD11c-reporter transgenes as markers of DCs, and of CD11c-DTR mice to 'selectively' deplete them [59,61]. Other markers that have been used to track macrophages and DCs in mice include F4/80, CD11b and MHC II, but they have also turned out to be nonspecific [57]. Too little attention has been paid to the expression of antimicrobi‐ al effector molecules, such as lysozyme, which is highly secreted by monocytes and macro‐ phages, but only weakly expressed, if at all, by DCs [62]. Part of the confusion may also result from the flexibility and plasticity of macrophages and from the presence of resident and migratory activated DCs in the same organ [63].

**5. Discriminating between DCs and macrophages**

64 Current Trends in Atherogenesis

es, but are in fact expressed by both [58] (figure 5).

It has become clear that DCs, especially DCs from myeloid origin, are very heterogeneous, representing several subtypes with a common origin, but different anatomical locations (lymphoid organs vs. non-lymphoid organs), function and phenotype. Moreover, there is al‐

The distinction of the differences between macrophages and the heterogeneous family of DCs is notoriously difficult and complicated by the plasticity of both cell types [55]. Mono‐ cytes that exit the blood and enter tissues under inflammatory conditions can differentiate to macrophages, but also to DCs that share several phenotypic features and functions, making it difficult to unambiguously define macrophages and DCs as individual entities [56]. In ad‐ dition, resting peripheral monocytes, obtained from mouse peritoneal cavity lavage, repre‐ sent an immature population, capable of further differentiation along either the dendritic or the macrophage pathway, depending on the type of stimuli (cytokines, growth factors) they receive [10]. Furthermore, many DC subsets are not clearly defined and it is absolutely nec‐ essary to bear in mind that different groups use different methods to identify and character‐ ize DCs [57]. Often, the starting populations are preselected based on randomly defined expression levels of markers that were believed to be specific for either DCs or macrophag‐

**Figure 5.** Functional characteristics and surface markers of DCs and macrophages. Increasing evidence demonstrates an enormous overlap between what is considered a 'macrophage' and a 'DC'. Abbreviations: MHC II, major histocompatibil‐ *ity complex class II; BDCA-1, blood dendritic-cell antigen-1; DC-SIGN, dendritic cell-specific ICAM-3-grabbing non-integrin.*

Consequently, confusion in distinguishing between macrophages and DCs has been – at least in part – caused by the use of nonspecific cell surface markers, such as CD11c. In addi‐

so a very close relationship between myeloid DCs and macrophages (figure 5).

The confusion could be possibly resolved if the appropriate reflections are considered [57]. For example, the correctness of CD11c to identify DCs depends on the anatomical site in question. In the spleen and lymph nodes, mononuclear phagocytes with high expression levels of CD11c – though not those with low or intermediate CD11c – appear to be DCs rath‐ er than macrophages. Accumulating evidence confirms that spleen and lymph node DCs are functionally different from macrophages, do not originate from differentiating monocytes, and share fewer characteristics with monocytes than macrophages [64,65,66]. However, in the lung, high levels of CD11c are expressed on macrophages [67,68], and there are many other anatomical locations apart from the lymphoid organs where macrophages are CD11cpositive. It has been proposed many times that the same set of markers that allows us to dis‐ criminate between DCs and macrophages in lymphoid organs, can also be used in nonlymphoid organs, but it has become clear that this assumption is not correct.

Recent *in vivo* experiments in mice have increased our understanding of the development and functions of DC and macrophage subsets [69,70,71]. However, despite this progress in mice, corresponding human subsets are yet to be characterized. Until now, there is no mor‐ phologic or protein marker of macrophages or DCs which is unambiguous. Moreover, a sin‐ gle set of markers cannot be assumed to apply to all stages of cell differentiation and activation. In conclusion, there is insufficient knowledge to make definitive claims about any marker combination, particularly in non-lymphoid compartments.

If the distinction between DCs and macrophages cannot be made based on morphological features, can it be based on function? Several criteria to define DCs include the property of DCs to localize in the T cell zone of lymphoid organs where they can stimulate T cells, as well as their ability to migrate and carry antigen [72,73]. In contrast, macrophages are best defined by their phagocytic activity and are generally considered as tissue-resident cells. However, recent studies show that macrophages can also migrate and that Langerhans cells (i.e. DCs from the skin and mucosa that carry large Birbeck granules) are not important for T cell priming [74]. In addition, some macrophage subtypes, such as microglia, show only poor phagocytic capacity [57]. Taken together, there is no good functional criterion to define macrophages and monocyte-derived DCs (figure 5), since they represent not just two differ‐ ent cell populations, but various cell subtypes. As they are derived from a common precur‐ sor, it is really hard to fully identify macrophages and DCs as two separate entities.

be caused by impaired DC differentiation from bone marrow progenitors. Until now, it remains unclear why plasma Flt3L levels are lowered in CAD. Other possible explana‐ tions for the decrease of circulating DC subsets in CAD patients include DC activation re‐ sulting in enhanced migration or in loss of subset markers, drug-induced changes, or

Dendritic Cells in Atherogenesis: From Immune Shapers to Therapeutic Targets

http://dx.doi.org/10.5772/52900

67

The finding that blood DCs are decreased in CAD patients and that atherosclerotic arteries display a marked increase in the number of DCs suggest the involvement of DCs in the pathogenesis of atherosclerosis. Yet, the exact role of DCs in atherogenesis has not been fully clarified. Moreover, increasing evidence points to different behaviour of DC subsets in the initiation and progression of the disease. We have recently demonstrated *in vitro* that mDCs in CAD operate in a normal way, whereas pDCs from CAD patients are not only reduced in

Most evidence points to a proatherogenic role for mDCs. Apolipoprotein E (ApoE)/IL-12 dou‐ ble knockout mice develop smaller atherosclerotic lesions than ApoE deficient (ApoE-/-) mice, illustrating the proatherogenic effect of IL-12, which is the main cytokine secreted by mDCs [85]. Moreover, daily IL-12 administration promotes atherosclerosis in ApoE-/- mice [86]. Be‐ cause mDCs from CAD patients are still able to mature [75], it is plausible that the blood mDCs that are activated by atherosclerosis-favouring factors in the circulation migrate to the athero‐ sclerotic plaque or the lymph nodes attached to the atherosclerotic wall segments. Once ar‐ rived, they might initiate and maintain the inflammatory response by continuous T-cell stimulation. Nevertheless, DCs are not only implicated in the immune response in atheroscle‐ rosis, they are also involved in cholesterol homeostasis. A recent study using a mouse model in which the receptor for diphtheria toxin was expressed under the CD11c promoter (CD11c-

mia [87]. The latter indicates that DCs are important in regulating the accumulation of lipids during the earliest stages of plaque formation. In contrast, enhancement of the life span and immunogenicity of DCs by specific overexpression of the anti-apoptotic gene hBcl-2 under the control of the CD11c promoter was associated with an atheroprotective decrease in plasma cholesterol levels, neutralizing the proatherogenic signature of enhanced T cell activation, a Th1 and Th17 cytokine expression profile, and elevated production of T-helper 1–driven IgG2c autoantibodies directed against oxidation-specific epitopes. -As a net result, there was no ac‐

It is not yet clear whether pDCs are proatherogenic or atheroprotective. PDCs might be in‐ volved in plaque destabilization, as they have the unique ability of producing large amounts of type I IFNs. This cytokine exerts strong antiviral effects, but more importantly, it induces marked upregulation of tumour necrosis factor (TNF)-related apoptosis-inducing ligand

ening the scaffold of the lesion and rendering the plaque vulnerable [88]. In addition, nucleoti‐ des released from necrotic or apoptotic cells can induce IFN-α production by pDCs in the presence of antimicrobial peptides released from inflammatory cells [89]. Plaque-residing pDCs have also been shown to respond to CpGs (containing motifs typically found in microbi‐

T cells, which might lead to killing of plaque-resident cells, potentially weak‐

cDCs resulted in enhanced cholesterolae‐

increased DC turnover, and are reviewed elsewhere [7].

number, but also seem to be functionally impaired [75].

DTR) showed that (transient) depletion of CD11c+

celeration of atherosclerotic plaque progression [87].

(TRAIL) on CD4+

#### **6. Pro-and anti-atherogenic properties of various DC subtypes**

We and others discovered a profoundly altered circulating DC compartment in patients with coronary artery disease (CAD), the clinical manifestation of atherosclerosis, as com‐ pared to healthy donors [75,76,77,78,79,80]. In 2006, we reported for the first time a decrease in circulating DC precursors (BDCA-1+ mDCs, BDCA-2+ pDCs) in CAD patients by flow cy‐ tometry. CAD was determined by angiography and defined as more than 50% stenosis in one or more coronary arteries [77]. In parallel, Yilmaz et al. [79] found a marked reduction in mDC precursors in CAD patients, though the decline in pDCs was less pronounced. Next, we studied whether the lower blood DC counts in CAD patients were related to the extent of atherosclerosis (one- versus three-vessel disease) or type (stable versus unstable angina pectoris) of CAD. Again, we observed significantly lower relative and absolute numbers of pDCs and mDCs in patients with coronary atherosclerosis [78]. Interestingly, the overall lin‐ eage-negative HLA-DR-positive blood DCs, which also include other blood DCs (such as BDCA-3+ ) or more mature blood DCs, confirmed the decline of BDCA+ DC precursors. How‐ ever, the counts of circulating DCs dropped to the same extent in three groups of CAD pa‐ tients, irrespective of the number (one or three) of affected arteries or the type (stable or unstable) of angina [78]. Consistent with our results, Yilmaz and colleagues [79] reported no differences between clinically stable or unstable CAD. Yet, in a later and more extended study with a cohort of 290 patients, in which a more refined 'CAD score' was used to classi‐ fy patients, they found that the numbers of pDCs, mDCs, and total DCs decreased when the extent of coronary atherosclerosis increased [80].

Besides flow cytometric studies, we performed immunohistochemical analyses demon‐ strating increased intimal DC counts with evolving plaque stages, in close relationship with lesional T cells [81]. These findings strongly suggest that blood DCs migrate from the circulation to the atherosclerotic lesion, possibly attracted by chemokines produced by the inflammatory infiltrate in the plaque, and subsequently stimulate T cell proliferation [7]. However, it is unlikely that accumulation of DC into a single tissue site is responsi‐ ble for the major changes in the number of circulating DCs in CAD [12]. Possibly, DCs may leave the blood to migrate into lymphoid tissues in response to systemic inflammato‐ ry activation, which redirects trafficking and compartmentalization of antigen-presenting DCs as well as lymphocytes. Indeed, it has been mentioned that DC numbers of lymph nodes attached to atherosclerotic wall segments exceed those in lymph nodes attached to non-atherosclerotic arteries [7]. The declined circulating DC numbers in atherosclerosis might also be the result of impaired differentiation from bone marrow progenitors. Inter‐ estingly, we recently showed that plasma Flt3 ligand (Flt3L) concentrations were reduced in CAD patients [75]. Flt3L is a major cytokine involved in both pDC and mDC develop‐ ment from haematopoietic stem cells and their release from the bone marrow [82,83,84]. As plasma Flt3L correlated with blood DC counts, the reduced blood DCs in CAD might be caused by impaired DC differentiation from bone marrow progenitors. Until now, it remains unclear why plasma Flt3L levels are lowered in CAD. Other possible explana‐ tions for the decrease of circulating DC subsets in CAD patients include DC activation re‐ sulting in enhanced migration or in loss of subset markers, drug-induced changes, or increased DC turnover, and are reviewed elsewhere [7].

ent cell populations, but various cell subtypes. As they are derived from a common precur‐

We and others discovered a profoundly altered circulating DC compartment in patients with coronary artery disease (CAD), the clinical manifestation of atherosclerosis, as com‐ pared to healthy donors [75,76,77,78,79,80]. In 2006, we reported for the first time a decrease

tometry. CAD was determined by angiography and defined as more than 50% stenosis in one or more coronary arteries [77]. In parallel, Yilmaz et al. [79] found a marked reduction in mDC precursors in CAD patients, though the decline in pDCs was less pronounced. Next, we studied whether the lower blood DC counts in CAD patients were related to the extent of atherosclerosis (one- versus three-vessel disease) or type (stable versus unstable angina pectoris) of CAD. Again, we observed significantly lower relative and absolute numbers of pDCs and mDCs in patients with coronary atherosclerosis [78]. Interestingly, the overall lin‐ eage-negative HLA-DR-positive blood DCs, which also include other blood DCs (such as

ever, the counts of circulating DCs dropped to the same extent in three groups of CAD pa‐ tients, irrespective of the number (one or three) of affected arteries or the type (stable or unstable) of angina [78]. Consistent with our results, Yilmaz and colleagues [79] reported no differences between clinically stable or unstable CAD. Yet, in a later and more extended study with a cohort of 290 patients, in which a more refined 'CAD score' was used to classi‐ fy patients, they found that the numbers of pDCs, mDCs, and total DCs decreased when the

Besides flow cytometric studies, we performed immunohistochemical analyses demon‐ strating increased intimal DC counts with evolving plaque stages, in close relationship with lesional T cells [81]. These findings strongly suggest that blood DCs migrate from the circulation to the atherosclerotic lesion, possibly attracted by chemokines produced by the inflammatory infiltrate in the plaque, and subsequently stimulate T cell proliferation [7]. However, it is unlikely that accumulation of DC into a single tissue site is responsi‐ ble for the major changes in the number of circulating DCs in CAD [12]. Possibly, DCs may leave the blood to migrate into lymphoid tissues in response to systemic inflammato‐ ry activation, which redirects trafficking and compartmentalization of antigen-presenting DCs as well as lymphocytes. Indeed, it has been mentioned that DC numbers of lymph nodes attached to atherosclerotic wall segments exceed those in lymph nodes attached to non-atherosclerotic arteries [7]. The declined circulating DC numbers in atherosclerosis might also be the result of impaired differentiation from bone marrow progenitors. Inter‐ estingly, we recently showed that plasma Flt3 ligand (Flt3L) concentrations were reduced in CAD patients [75]. Flt3L is a major cytokine involved in both pDC and mDC develop‐ ment from haematopoietic stem cells and their release from the bone marrow [82,83,84]. As plasma Flt3L correlated with blood DC counts, the reduced blood DCs in CAD might

pDCs) in CAD patients by flow cy‐

DC precursors. How‐

sor, it is really hard to fully identify macrophages and DCs as two separate entities.

**6. Pro-and anti-atherogenic properties of various DC subtypes**

) or more mature blood DCs, confirmed the decline of BDCA+

in circulating DC precursors (BDCA-1+ mDCs, BDCA-2+

extent of coronary atherosclerosis increased [80].

BDCA-3+

66 Current Trends in Atherogenesis

The finding that blood DCs are decreased in CAD patients and that atherosclerotic arteries display a marked increase in the number of DCs suggest the involvement of DCs in the pathogenesis of atherosclerosis. Yet, the exact role of DCs in atherogenesis has not been fully clarified. Moreover, increasing evidence points to different behaviour of DC subsets in the initiation and progression of the disease. We have recently demonstrated *in vitro* that mDCs in CAD operate in a normal way, whereas pDCs from CAD patients are not only reduced in number, but also seem to be functionally impaired [75].

Most evidence points to a proatherogenic role for mDCs. Apolipoprotein E (ApoE)/IL-12 dou‐ ble knockout mice develop smaller atherosclerotic lesions than ApoE deficient (ApoE-/-) mice, illustrating the proatherogenic effect of IL-12, which is the main cytokine secreted by mDCs [85]. Moreover, daily IL-12 administration promotes atherosclerosis in ApoE-/- mice [86]. Be‐ cause mDCs from CAD patients are still able to mature [75], it is plausible that the blood mDCs that are activated by atherosclerosis-favouring factors in the circulation migrate to the athero‐ sclerotic plaque or the lymph nodes attached to the atherosclerotic wall segments. Once ar‐ rived, they might initiate and maintain the inflammatory response by continuous T-cell stimulation. Nevertheless, DCs are not only implicated in the immune response in atheroscle‐ rosis, they are also involved in cholesterol homeostasis. A recent study using a mouse model in which the receptor for diphtheria toxin was expressed under the CD11c promoter (CD11c-DTR) showed that (transient) depletion of CD11c+ cDCs resulted in enhanced cholesterolae‐ mia [87]. The latter indicates that DCs are important in regulating the accumulation of lipids during the earliest stages of plaque formation. In contrast, enhancement of the life span and immunogenicity of DCs by specific overexpression of the anti-apoptotic gene hBcl-2 under the control of the CD11c promoter was associated with an atheroprotective decrease in plasma cholesterol levels, neutralizing the proatherogenic signature of enhanced T cell activation, a Th1 and Th17 cytokine expression profile, and elevated production of T-helper 1–driven IgG2c autoantibodies directed against oxidation-specific epitopes. -As a net result, there was no ac‐ celeration of atherosclerotic plaque progression [87].

It is not yet clear whether pDCs are proatherogenic or atheroprotective. PDCs might be in‐ volved in plaque destabilization, as they have the unique ability of producing large amounts of type I IFNs. This cytokine exerts strong antiviral effects, but more importantly, it induces marked upregulation of tumour necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) on CD4+ T cells, which might lead to killing of plaque-resident cells, potentially weak‐ ening the scaffold of the lesion and rendering the plaque vulnerable [88]. In addition, nucleoti‐ des released from necrotic or apoptotic cells can induce IFN-α production by pDCs in the presence of antimicrobial peptides released from inflammatory cells [89]. Plaque-residing pDCs have also been shown to respond to CpGs (containing motifs typically found in microbi‐ al DNA) leading to enhanced IFN-α expression. This process amplifies inflammatory TLR-4, TNF-α, and IL-12 expression by mDCs, and correlates with plaque instability [90]. A recent study in ApoE-/- mice reported that administration of a plasmacytoid dendritic cell antigen-1 (PDCA-1) antibody to deplete pDCs protected from lesion formation [91], demonstrating that pDCs indeed exert proatherogenic functions during early lesion formation. In contrast, pDC depletion by administration of the 120G8 monoclonal antibody promoted plaque T-cell accu‐ mulation and exacerbated lesion development and progression in LDLr⁻/ ⁻ mice [92]. PDC de‐ pletion was accompanied by increased CD4⁺ T-cell proliferation, IFN-γ expression by splenic T cells, and plasma IFN-γ levels, pointing to a protective role for pDCs in atherosclerosis. Thus, the exact role of pDCs in atherosclerosis remains to be further unravelled.

could be used for vaccination as well, thereby avoiding the side effects of direct vaccination with oxLDL [108]. A series of studies have already used pulsed DCs as an immunotherapy for atherosclerosis in mice, however, results were not always consistent. Repeated injection of LDLr-/- mice with oxLDL-pulsed mature DCs resulted in attenuation of lesion develop‐ ment with a decreased amount of macrophages and increased collagen content, contributing to a more stable plaque phenotype [109]. Moreover, a similar approach was carried out us‐ ing mice expressing the full-length human ApoB100 in the liver and humanized lipoprotein profiles [110]. Those mice were repeatedly injected with mature DCs that were incubated with IL-10 and ApoB100, prior to the initiation of a Western diet. The immunosuppressive cytokine IL-10 was used to induce tolerogenic DCs [110]. This approach resulted in attenua‐ tion of atherosclerotic lesion development in the aorta, which was associated with decreased cellular immunity to ApoB100. Also, decreased Th1 and Th2 responses most likely due to enhanced regulatory T cell (Treg) expansion were observed [110]. In contrast, subcutaneous injection of DCs that were simultaneously pulsed with LPS and MDA-LDL into ApoE-/- mice at frequent intervals during lesion formation caused a significant increase in lesion size in the aortic root [111]. These differential effects may be due to different forms of antigen pre‐ sentation leading to qualitatively different immune responses. Apart from oxLDL, DCs might also be pulsed *ex vivo* by cultivating them with a total extract or suspension of athero‐ sclerotic plaque tissue, for example, from patients undergoing carotid endarterectomy [95,108] (figure 6). A major advantage of such a therapy, where a patient is vaccinated with its own DCs pulsed by its own antigens is the efficiency, because it would imitate events as

Dendritic Cells in Atherogenesis: From Immune Shapers to Therapeutic Targets

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69

**Figure 6.** Promising areas for further research to treat immune-mediated diseases, such as atherosclerosis. Immuniza‐ tion of patients with autologous, monocyte-derived DCs that are loaded with appropriate antigens *ex vivo*. This ap‐

they occur in plaques *in situ* in the patient.

proach has already been proven successful in cancer and HIV patients.
