**5. Inflammatory and resident cells**

A comparison of the number of pericyte-like cells in atherosclerotic lesions with the number of pericyte-like cells in undiseased intima revealed that in atherosclerotic lesions, the number of the cells possessing 3G5 antigen is much lower than that in undiseased intima (Figure 5).

**Figure 5.** Visualization of 3G5+ cells in normal (undiseased) intima (**A**), in fatty streak (**B**), and in atherosclerotic pla‐

This could suggest that the number of pericyte-like cells in atherosclerotic lesions decreases. However, the results obtained in experiments with the use of antipericyte antibody 2A7, which recognizes another pericyte antigen, argue with such a suggestion. 2A7+ cells were found to appear in atherosclerotic plaques, while 2A7+ cells are absent in normal intima (Table 1). Anti-2A7 represents an antibody against melanomaassociated high-molecular-weight antigen (HMW-MAA), which is a chondroitin sulfate proteoglycan (also termed as melanoma proteoglycan), which is also present on pericytes in the areas of active angiogenesis (in granulation tissues in healing wounds, synoviitis, etc.) [61]. 2A7 is expressed by "activated"

The above-described findings led to a question: are 3G5 and 2A7 pericytic antigens expressed by the same cell or by different cell types? In order to answer this question, a culture of human brain pericytes was used for experiments. It has been found that in human brain pericytes, about 40% of cells express 3G5 antigen and 80% of cells express 2A7 antigen. Simultaneous staining for both antigens revealed 80% of positively stained cells indicating that all cells having 3G5 antigen expressed 2A7 antigen; however, there was a population of pericytes that expressed only 2A7 antigen. As was mentioned above, 2A7 and 3G5 antigens were described on the cells in different functional states: 3G5 antigen is typical for "quiescent" but 2A7 for "activated" pericytes, respectively. Activated 2A7-positive pericytes are capable of intense replication. It can be hypothesized that most of the cultured pericytes are "activated" (as a result of serum stimulation); therefore, only a part of these preserves 3G5 antigen, which is typical

ques (**C**) by means of immunofluorescent analysis.

210 Muscle Cell and Tissue

pericytes capable of proliferating [61].

for quiescent pericytes.

Immunocytochemical analysis of the cellular composition of the intimal layers showed that the cell population of the muscular-elastic layer is homogeneous, consisting predominantly of smooth muscle cells. These cells are similar to the medial smooth muscle cells, the majority of which react with antimuscle α-actin antibodies. The proteoglycan-rich layer of the intima is populated by resident and inflammatory cells. In the atherosclerotic lesions, the proportion of resident cells expressing smooth muscle α-actin is similar to that in uninvolved intima, while the proportion of inflammatory cells increases, but these cells do not become predominating. Besides the smooth muscle cell antigen, resident intimal cells express pericyte antigens as well as the macrophage-associated antigen CD68.

What are the specific features of resident intimal cells? Morphological heterogeneity of intimal population consisting of processed cells of various shapes was described by Langhans [27], Schonfelder [62], Schlekunov [30, 63], Khavkin [64], Geer and Haust [19], and others. Elongated bipolar cells typical of the media predominate in the population of the muscular-elastic layer. At the same time, the cells of proteoglycan layer markedly vary in shape: they are elongated and stellate, with a variety of intermediate shapes. The presence of processed star-like cells [27] is the major characteristic of the proteoglycan-rich layer.

The morphological forms of resident subendothelial cells (elongated and stellate) have been described in primary cultures of enzyme-isolated cells from normal and atherosclerotic human aorta [65]. Elongated cells have a long body without processes or with small side processes. They are packed in compact cell layers, express α-actin, and have a well-developed contractile apparatus [65]. Stellate (pericyte-like) cells have a round body with three or more processes. They are unevenly distributed in the loose connective-tissue matrix of the intima. In addition to α-actin, some juxtaluminally located cells express 3G5 and 2A7 pericytic antigens and CD68 macrophage-associated antigen [13], a scavenger receptor [39]. ln atherosclerotic lesions, the cytoplasm of these cells is filled with lipids (foam cells) [66]. Electron microscopic studies revealed considerable numbers of synthetic organelles in these cells [14]. In the atherosclerotic lesions, the number of stellate cells increases sixfold while the total number or cells and the number of elongated cells increase only twofold. Thus, both quantitative and qualitative changes occur in cellular composition of the intima underlying atherosclerotic lesions.
