Myofibroblasts Are Not Characteristic Features of Keloid Lesions

Abstract

Keloids are disfiguring, scar-like lesions that are challenging to treat, with low response rates to current interventions and frequent recurrence. It has been widely reported that keloids are characterized by myofibroblasts, specialized contractile fibroblasts that express alpha-smooth muscle actin (α-SMA). However, evidence supporting a role for myofibroblasts in keloid pathology is inconclusive, with conflicting reports in the literature. This complicates development of more effective therapies, as the benefit of interventions targeting myofibroblasts is unclear. This study was undertaken to determine whether myofibroblasts can be considered characteristic of keloids.

Methods: Myofibroblasts in tissue sections from keloids, hypertrophic scars (HTSs), and normal skin were localized by α-SMA immunostaining. Expression of α-SMA mRNA (ACTA2 gene) in normal skin and keloid tissue, and in fibroblasts from normal skin, keloid, and HTSs, was measured using quantitative polymerase chain reaction.

Results: Normal skin did not exhibit α-SMA-expressing myofibroblasts, but myofibroblasts were identified in 50% of keloids and 60% of HTSs. No significant differences in ACTA2 expression between keloid and normal skin tissue were observed. Mean ACTA2 expression was higher in HTS (2.54-fold, P = 0.005) and keloid fibroblasts (1.75-fold, P = 0.046) versus normal fibroblasts in vitro. However, α-SMA expression in keloids in vivo was not associated with elevated ACTA2 in keloid fibroblasts in vitro.

Conclusions: Despite elevated ACTA2 in cultured keloid fibroblasts, myofibroblast presence is not a consistent feature of keloids. Therefore, therapies that target myofibroblasts may not be effective for all keloids. Further research is required to define the mechanisms driving keloid formation for development of more effective therapies.

Fig. 1.

Localization of myofibroblasts in normal human skin, keloids, and HTSs. Shown are representative sections of normal human skin (A, B), keloids (C, F), and HTSs (G, H) stained for α-SMA (red) and collagen IV (green). Nuclei were counterstained using 4′,6-diamidino-2-phenylindole dihydrochloride (blue). Examples of myofibroblasts staining positive for α-SMA and not colocalized with collagen IV are indicated by white arrows (E–H). The scale bar in A is the same for all panels (200 μm).

Fig. 2.

Expression of ACTA2 mRNA in normal skin (black bar) and keloid (red bar) tissue samples. Expression of human ACTA2, which encodes α-SMA, was measured using qPCR. Expression levels were normalized to mean expression in normal skin, and the group means + SDs were plotted. No significant difference was observed between mean ACTA2 mRNA expression in normal human skin compared with keloid lesions (N = 3 samples each). SD indicates standard deviation.

Fig. 3.

Expression of ACTA2 mRNA in fibroblasts cultured from normal skin (black bar; N = 11), keloids (red bar; N = 12), and hypertrophic scars (blue bar; N = 3). Expression of human ACTA2, which encodes α-SMA, was measured using qPCR. Expression levels were normalized to mean expression in normal fibroblasts, and the group means + SDs were plotted. Mean ACTA2 expression was significantly higher in keloid and HTS-derived fibroblasts compared with normal skin fibroblasts; P values are indicated on the plot. No significant difference was observed between mean ACTA2 expression in HTS fibroblasts compared with keloid fibroblasts. SD indicates standard deviation.

Fig. 4.

Mean ACTA2 expression in keloid fibroblasts in vitro. Expression of ACTA2 in fibroblasts cultured from keloids containing α-SMA-expressing myofibroblasts (black bar; N = 4) was compared with fibroblasts cultured from keloids devoid of α-SMA-expressing myofibroblasts (red bar; N = 7). No significant difference in ACTA2 expression between these two keloid fibroblast populations was observed. Plotted are mean normalized expression levels + SDs. SD indicates standard deviation.