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Keloids – the sebum hypothesis revisited
Medical Hypotheses (2002) 58(4), 264±269 & 2002 Published by Elsevier Science Ltd doi: 10.1054/mehy.2001.1426, available online at http://www.idealibrary.com on
1
Keloids ± the sebum hypothesis revisited E. P. Fong,1 B. H. Bay2 1
Department of Surgery, University of Malaya Medical Centre, West Malaysia; 2Department of Anatomy, National University of Singapore, Singapore
Summary The aetiology of the keloid scar has not been completely elucidated. Numerous hypotheses have been proposed in the past to explain the unusual characteristics of the keloid scar. While we do know that there is excessive and ongoing collagen-deposition, the exact triggering stimulus is a subject of conjecture. We present some of our photographic records of keloids and electron microscopic findings of keloid edges and reiterate the sebum hypothesis. We also attempt to explain the features of keloids in the light of the present knowledge of immunology and cell biology. & 2002 Published by Elsevier Science Ltd
INTRODUCTION The keloid scar continues to be a complex and poorly understood subject. The main problem faced by researchers is the lack of an animal model, as keloids affect only humans. At present, evidence from researchers show that transforming growth factor-beta (TGFb), is one of the main cytokines responsible for stimulating the synthesis of matrix proteins, including collagen and fibronectin by fibroblasts (1±3), while suppressing proteases (4). TGFb is released in its latent form by platelets, lymphocytes, macrophages, endothelial cells and fibroblasts (5,6), and has been co-localized with type I and VI collagen at the expanding edge of keloid tissue (7). The most important question remains unanswered. What is the triggering stimulus for the cascade of events in the formation of excessive collagen and extracellular matrix in keloids? A stimulus that is either constant or resurfaces every so often to cause the keloid to progress and enlarge, must be responsible.
Received 11 January 2001 Accepted 25 May 2001 Correspondence to: Dr Eileen Fong MBBS FRCS(Edin), Department of Surgery, University of Malaya Medical Centre, Pantai Valley, Kuala Lumpur 59100, West Malaysia. Fax: 603 7955 7740; E-mail: [email protected]
264
A number of hypotheses have been proposed in the past to explain the keloid phenomenon. Mowlem (8) first suggesting that keratin from either the hair follicle or sebaceous gland could be the initiating stimulus. Yagi et al. (9), observing the absence of keloid in areas devoid of sebaceous glands, proposed sebum to be the responsible factor. They attempted desensitization of known keloid formers with intradermal injection of sebum prior to keloid excision. However, their results were hampered by poor follow-up. Crockett (10) and Peacock et al. (11) believed skin tension to be the stimulus for the excessive production of collagen, citing sites of keloid predilection in areas of so-called tension on the chest, deltoid and back. Skin tension was believed to be the reason why keloid occurred in young people and was almost absent in the elderly. This led Harvey-Kemble (12) to propose intralesional excision of keloids, leaving a rim behind to serve as a splint to counter tension. Hunt et al. (13) considered the hypoxic nature of wounds to be the stimulus for fibroblasts to produce collagen. They noted elevated lactate dehydrogenase enzyme levels in extracellular fluid of wounds associated with anaerobic metabolism. Kischer et al. (14) observed most microvessels in hypertrophic scans and keloids to be occluded, owing to an excess of endothelial cells that bulged into the lumen. They attributed the occlusion to either endothelial cell proliferation or perivascular myofibroblast contraction.
Keloids
That the keloid fibroblast is a mutant monoclonal strain with abnormal increase in collagen synthesis was proposed by Cohen et al. (15) and Abergel et al. (16). However, Russel et al. (17) and Ala-Kokka et al. (18), using radioactive-labeled proline to trace collagen biosynthesis, found that keloid fibroblast produced similar quantities of collagen relative to normal controls. Moreover, Fleischmajer et al. (19) showed that normal human skin fibroblasts are a heterogeneous population of cells which can exhibit up to 20-fold differences in their collagen synthesis, and this heterogeneity is conserved over multiple population doublings. Cohen et al. (20) also proposed that histamine may have a stimulatory effect on collagen synthesis in keloid and hypertrophic scars, noting the histamine content of keloid tissue to be greater than that of hypertrophic scars. Russell et al. (21) noted that histamine caused a marked increase in growth of cultured fibroblast from both normal and keloid tissue, with no difference in growth rate. Mast cells in keloids, responsible for histamine production by degranulation, have been noted by researchers (22,23). In addition to having a stimulatory effect on fibroblast collagen synthesis, Marks et al. (24) also noted the mitogenic effect on microvascular endothelial cells. Among the proponents of the immune-response hypothesis, Oluwasanmi (25) observed an increase in the number of plasma cells and immunoglobulin G deposition along keloid collagen fibres, and suggested that individuals with high levels of circulating immunoglobulin (Ig) are more likely to produce keloids when traumatized. Cohen et al. (26) noted tissue IgG in keloids to be raised compared to normal skin and scar, suggesting a local immune response. No significant difference in serum levels of IgG and no correlation between HLA phenotype and keloid formation was noted. Kischer et al. (27) noted increased levels in all IgG, IgA and IgM in hypertrophic scar and keloid compared to normal skin. No one Ig was significantly raised over the others, suggesting leakage of plasma proteins from microvasculature, possibly due to histamine. CLINICAL EVIDENCE Over the years, we have kept a photographic record of keloids. Despite their variable features, they exhibit a unifying characteristic which prompts us to reiterate the sebum hypothesis. Figure 1 shows a typical earlobe keloid developing as a result of trauma from piercing. What is unusual is the fact that the other ear-hole created in close proximity and at the same time has healed normally with an epithelialized tract. In the other ear of the same patient (not shown), both earholes have healed in the normal manner as well. This puts into question the long-held belief of regional & 2002 Published by Elsevier Science Ltd
265
Fig. 1 Upper ear-hole has formed a keloid while lower ear-hole has healed normally.
Fig. 2 BCG scar at 12 years-of-age forming a keloid. BCG scar at neonate (arrow) heals normally.
susceptibility in keloids, the phenomenon which is often attributed to differences in skin tension. Figure 2 shows a young lady in her thirties who has formed a keloid on her left deltoid from a BCG vaccination given at age 12. Interestingly, a previous BCG vaccination given at neonate, just above the present keloid, has healed normally (see arrow). She also has `spontaneous keloids' on her chest, presumably from previous forgotten incidences of acne (not shown). We have encountered several other examples of this phenomenon ± where the BCG scar at neonate is normal while that given at 12 years-of-age (according to national protocol) has developed into a keloid. Medical Hypotheses (2002) 58(4), 264±269
266 Fong and Bay
Fig. 4 Normal thyroidectomy scar. Keloid formation from puncture site for drainage tube.
Fig. 3 Keloid formation following carbon dioxide laser to tattoo on left deltoid.
Figure 3 shows a young man in his twenties who has extensive tattoo covering his deltoid, chest and back regions. He has had carbon dioxide laser therapy to the tattoo on his left deltoid by another surgeon in an attempt to remove the tattoo marks. Of interest is the fact that he has developed a progressively enlarging keloid on his left deltoid from the carbon dioxide laser, while none of his tattoo-inflicted wounds have formed a keloid. This would seem to add another piece of evidence in support of the sebaceous gland as the responsible stimulus, and not keratin from keratinocytes or hair follicles, as some theories propose. It is a commonly accepted observation that keloids are rare at extremes of life and uncommon before puberty. Piecing the evidence together, it would seem that the occurrence of keloids is an affair dependent on some structure in the skin which is regulated by the sex hormone. The sebaceous gland is a likely candidate. Activity and size of the sebaceous gland is age-related. Sebum excretion increases at puberty and gradually decreases in adulthood after menopause. The active androgen at tissue level responsible for stimulating the sebaceous gland is 5-alpha-dihydrotestoterone, the active metabolite of testosterone and androstenedione (28). Both circulating hormone levels increase at puberty at a time when keloid makes its debut. While the pilosebaceous gland is ubiquitous in its distribution in the body (apart from the palm and sole), it differs in characteristics in different regions of the body Medical Hypotheses (2002) 58(4), 264±269
(29). Vellus follicles are smaller with tiny hairs and are mainly found on the face. The sebaceous follicles have large multilobular glands with thin filmsy hair and are found in acne-prone areas on the face, chest and upper back. The terminal beard follicle has stiff, thick hair and smaller sebaceous gland. The distribution of the oftenseen and well-described phenomenon of `spontaneous keloids' on the back, chest, shoulder and face again suggests the sebaceous gland. These spontaneous keloids have often been thought to occur from previously forgotten incidences of acne ± a condition associated with overactive sebaceous glands. Figure 4 shows a lady with a thyroidectomy scar which has healed in an interesting manner. While the scar on her neck has healed normally, the scar on her chest following a single puncture for the drainage tube has formed a progressively enlarging keloid, which is growing downwards towards her chest. Again, the keloid scar has formed in an area where the sebaceous follicles are perhaps more numerous and voluminous.
DISCUSSION Martin and Muir's (30) work on the role of lymphocytes in wound healing showed a much greater concentration of lymphocytes in keloids as opposed to hypertrophic scars and normal scars. The lymphocytes were persistent in keloids as old as 9 years, suggesting a continuous stimulus. In normal scars, lymphocytes were seen until about 4±5 weeks, and then disappeared rapidly. Also notable is the absence of B-lymphocytes. A similar finding was noted by Appleton et al. (31). They noted numerous apoptotic cells at the interface between dermis and keloid ± a feature characteristic of cell-mediated immune attack. In addition, other nonlymphoid inflammatory cells were seen. & 2002 Published by Elsevier Science Ltd
Keloids
Proposal of a delayed-type hypersensitivity reaction It would seem likely that the chain of events starts with the initial disruption to the pilosebaceous structure, causing leakage of sebum. This antigenic stimulus in sensitive individuals triggers T-lymphocyte recognition and proliferation of antigen-specific T-lymphocytes similar to a delayed-type hypersensitivity reaction (32). The result is clonal expansion of T-lymphocytes migrating to the site of stimulus. Activated T-lymphocytes secrete TGFb, in addition to other cytokines which stimulate the inflammatory response. These cytokines act on macrophages, which serve a phagocytic function, inflammatory leucocytes, which are present at the early stage and mast cells, which secrete histamine and heparin causing features of pruritis and microvascular endothelial cell proliferation (Fig. 5). Some of the progeny develop into antigen-specific memory T cells, which initiate larger secondary immune responses upon subsequent exposure to the antigen. Because the enlarging keloid edge entraps and disrupts adjacent pilosebaceous units, each encounter with the antigen triggers the cascade of events, causing the periodic features of activity and pruritis in keloids. Hence the occurrence of the earlobe keloid from a single prick is a chance phenomenon, depending on the probability of dispersing the antigen. The sebum hypothesis explains the distribution and behavior of keloids. Keloids affect only humans, as animals do not have comparable sebaceous glands. They do not occur on the palms and soles since these areas are devoid of sebaceous glands. Keloids occur in adolescence and early adulthood when sebum production is maximal. Corticosteroids injected into keloids suppress the cellmediated immune response to antigenic stimulus, accounting for the apparent benefit. Repeated injections
Fig. 5 Delayed-type hypersensitivity reaction.
& 2002 Published by Elsevier Science Ltd
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are required for suppression. Superficial X-ray therapy may confer some benefit due to its known sebumreducing effects (33). Electron microscopy of keloid edges To this end, we have examined the keloid edge in detail. Electron microscopy of a number of active keloid edges show the presence of sebaceous glands. To attempt to look for disruption of the gland right at the invading edge is akin to looking for a split-second event in time. Figure 6 is an electron microscopy of a keloid edge, with whorls of collagen fibres and adjacent sebocyte from sebaceous gland nearby. It would seem highly likely for the enlarging mass of collagen fibres to encroach on to the sebocyte and disrupt it, dispersing sebum into the surrounding tissue and thereby triggering the cascade of events leading to keloid formation. In addition, macrophages are seen with secondary lysosomes effecting phagocytic function (Fig. 7) and mast cells are evident in significant numbers (Fig. 8) in all the active keloid edges examined. Cellular immunology The major type of T cell-mediated immune response in the skin is the delayed-type hypersensitivity reaction. T helper lymphocytes recognize only short sequences of peptide antigens attached to proteins that are encoded in the
Fig. 6 Electron microscopy of keloid edge with adjacent sebocyte. Medical Hypotheses (2002) 58(4), 264±269
268 Fong and Bay
Fig. 7 Macrophages at keloid edge effecting their phagocytic function.
According to Burnet's Clonal Selection Hypothesis, the development of antigen-specific clones of lymphocytes occurs prior to and independent of exposure to antigen (35). The lymphocyte repertoire is extremely large and the mechanism of T lymphocyte loss of self-tolerance in specific clones remains a subject of conjecture. It has been suggested that the loss of self-tolerance may result from abnormal selection of self-reactive clones to T lymphocytes (32). In normal circumstances, these self-reactive clones of immature T cells are negatively selected or deleted in the thymus. Although the thymus involutes after puberty, T lymphocytes continue to arise from bone marrow stem cells in adults and self-reactive clones continue to be negatively selected, since normal individuals remain selftolerant throughout their lives. A possible mechanism for this self-tolerance or T cell clonal anergy is co-stimulator deficiency of the antigen-presenting cells. In situations of tissue injury and inflammation or microbial infection, resident antigen-presenting cells are activated, leading to an increased expression of co-stimulators, loss of selftolerance and local autoimmune reactions. Another possible explanation for this loss of self-tolerance in tissue injury is the release of anatomically sequestered antigens from their normally concealed locations. Known clinical examples of this is the phenomena of post-traumatic uveitis and orchitis following vasectomy (36,37). CONCLUSION The sebum hypothesis, or perhaps more appropriately the sebocyte hypothesis, explains the behavior of keloids. The sebocyte as a self-antigen may result from abnormal selection of self-reactive clones of T lymphocytes, abnormal stimulation of T lymphocytes that are normally anergic to self-antigens or release of self-antigens that are normally inaccessible to the immune system. The keloid microenvironment is heterogenous and the influence of various factors are spatially related. Our evidences suggest a delayed-type hypersensitivity reaction, with the sebum or sebocyte being the persistent antigenic stimulus.
Fig. 8 Mast cells (single arrow) and lymphocyte (double arrows) at keloid edge.
major histocompatibility complex and expressed on the surface of antigen-presenting cells (32). Although the major component of sebum is lipid, sebocytes discharge by a holocrine mechanism, with entire cell disruption (34). It is possible that sebum constituents bind to and modify self-proteins within the sebocyte. Medical Hypotheses (2002) 58(4), 264±269
ACKNOWLEDGEMENTS We would like to thank Mr John A. Kompa, Lecturer in Multimedia Design, Temasek Polytechnic School of Design, Singapore, for his help with the illustration.
REFERENCES 1. Ignotz R. A., Massague J. Regulation of fibronectin and type I collagen mRNA levels by transforming growth factor-b. J Biol Chem 1987; 262: 6443±6447.
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2. Raghow R., Postlewaite A. E., Keski-Oja J., Moses H. L., Kang A. H. Transforming growth factor-b increases steady state levels of type I procollagen and fibronectin messenger RNAs posttranscriptionally in cultured human dermal fibroblasts. J Clin Invest 1987; 79: 1285±1288. 3. Shah M., Foreman D. M., Ferguson M. J. Control of scarring in adult wounds by neutralising antibody to transforming growth factor-b. Lancet 1992; 25: 213±214. 4. Overal C. M., Wrana J. L., Sodek J. Independent regulation of collagenase, 72 kDa-progelatinase and metalloproteinase inhibitor (TIMP) expression in human fibroblasts by transforming growth factor-b. J Biol Chem 1989; 264: 1860±1869. 5. Assoian R. K., Komoriya A., Meyers C. A. Transforming growth factor-beta in human platelets. J Biol Chem 1983; 258: 7155±7160. 6. Frolik C. A, Dart L. L., Meyers C. A. Purification and initial characterization of a type beta transforming growth factor from human placenta. Proc Natl Acad Sci USA 1983; 80: 3676±3680. 7. Pletonen J., Hsiao L., Joahkola S. Activation of collagen gene expression in keloids: co-localization of type I and VI collagen and transforming growth factor-beta I mRNA. J Invest Dermatol 1991; 97: 240±248. 8. Mowlem R. Hypertrophic scars. Br J Plast Surg 1951; 4: 113±120. 9. Yagi K. I., Dafalla A. A., Osman A. A. Does an immune reaction to sebum in wounds cause keloid scars? Beneficial effect of desensitisation. Br J Plast Surg 1979; 32: 223±225. 10. Crockett D. J. Hypertrophic scars. Br J Plast Surg 1964; 17: 245±253. 11. Peacock E. E., Madden J. W., Trier W. C. Treatment of keloids. Southern Med J 1970; 63: 755±759. 12. Harvey Kemble J. V. The management of scars, hypertrophic scars and keloids. Surgery 1988; 54: 1286. 13. Hunt T. K., Conolly W. B., Aronson S. B., Goldstein P. Anaerobic metabolism and wound healing: an hypothesis for the initiation and cessation of collagen synthesis in wounds. Am J Surg 1978; 135: 328±332. 14. Kischer C. W., Thies A. C., Chvapil M. Perivascular myofibroblast and microvascular occlusion in hypertrophic scars and keloids. Hum Pathol 1982; 13: 819±824. 15. Cohen I. K., Diegelmann R. F., McCoy B. Elevated collagen synthesis in cultured keloid fibroblasts. Surg Forum 1977; 28: 526±527. 16. Abergel R. P., Pizzurro D., Meeker C. A. et al. Biochemical composition of the connective tissue in keloids and analysis of collagen metabolism in keloid fibroblast cultures. J Invest Dermatol 1985; 84: 384±390. 17. Russell J. D., Witt W. S. Cell size and growth characteristics of cultured fibroblasts isolated from normal and keloid tissue. Plast Reconstr Surg 1976; 57: 207±212. 18. Ala-Kokko L., Rintala A., Savolainen E. Collagen gene expression in keloids: analysis of collagen metabolism and type I, III, IV and V procollagen mRNAs in keloid tissue and keloid fibroblast cultures. J Invest Dermatol 1987; 89: 238±244. 19. Fleischmajer R., Perlish J. S., Krieg T., Timpl R. Variability in collagen and fibronectin synthesis by scleroderma fibroblasts in primary culture. J Invest Dermatol 1981; 76: 400±403.
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20. Cohen I. K., Beaven M. A., Horakova Z., Keiser H. R. Histamine and collagen synthesis in keloid and hypertrophic scar. Surg Forum 1972; 23: 509±510. 21. Russell J. B., Russell S. B., Trupin K. M. The effect of histamine on the growth of cultured fibroblasts isolated from normal and keloid tissue. J Cell Physiol 1977; 93: 389±394. 22. Mukherjee A., Dey T. K., Saha K. C. Tissue mast cells in keloidal scars. Indian J Dermatol 1981; 26: 7±10. 23. Smith C. J., Smith J. C., Finn M. C. The possible role of mast cells (allergy) in the production of keloid and hypertrophic scarring. J Burn Care Rehabil 1987; 8: 126±131. 24. Marks R. M., Roche W. R., Czerniecki M., Penny R., Nelson D. S. Mast cell granules cause proliferation of human microvascular endothelial cells. Lab Invest 1986; 55: 289±294. 25. Oluwasanmi J. O. Keloids in the African. Clin Plast Surg 1974; 1: 179±195. 26. Cohen I. K., McCoy B. J., Mohanakumar T., Diegelmann R. F. Immunoglobulin, complement, and histocompatibility Ag studies in keloid patients. Plast Reconstr Surg 1979; 63: 689±695. 27. Kischer C. W., Shetlar M. R., Shetlar C. L., Chvapil M. Immunoglobulin in hypertrophic scars and keloids. Plast Reconstr Surg 1983; 71: 821±825. 28. Blauer M., Vaalasti A., Pauli S. L., Ylikomi T., Joensuu T., Tuohimaa P. Location of androgen receptor in human skin. J Invest Dermatol 1991; 97: 264±268. 29. Cunliffe W. J., Perera W. D. H., Thackray P., Williams M., Forster R. A., Williams S. M. Pilosebaceous duct physiology. III. Observations on the number and size of pilosebaceous ducts in acne vulgaris. Br J Dermatol 1976; 95: 153±156. 30. Martin C. W., Muir I. F. K. The role of lymphocytes in wound healing. Br J Plast Surg 1990; 43: 655±662. 31. Appleton I., Brown N. J., Willoughby D. A. Apoptosis, necrosis and proliferation. Possible implications in the etiology of keloids. Am J Path 1996; 149(5): 1441±1447. 32. Abbas A. K., Lichtman A. H., Pober J. S. Effector mechanism of T-cell-mediated immune response. In: Cellular and Molecular Immunology, Second Edn. City: W. B. Saunders, 1994. 33. Kirk J., Worthley B. W. Superficial radiotherapy in acne vulgaris. Australas J Dermatol 1971; 12(1): 10±17. 34. Thody A. J., Shuster S. Control and function of sebaceous glands. Physiol Rev 1989; 69(2): 383±416. 35. Burnet M. F. A modification of Jerne's theory of antibody production using the concept of clonal selection. Austr J Sci 1957; 20: 67±69. 36. Rahi A., Morgan G., Levy I., Dinning W. Immunological investigations in post-traumatic granulomatous and nongranulomatous uveitis. Br J Opthalmol 1978; 62(10): 722±728. 37. Roper R. J., Doerge R. W., Call S. B., Tung K. S., Hickey W. F., Teuscher C. Autoimmune orchitis, epididymitis and vasitis are immunogenetically distinct lesions. Am J Pathol 1998; 152(5): 1337±1345.
Medical Hypotheses (2002) 58(4), 264±269
1
Keloids ± the sebum hypothesis revisited E. P. Fong,1 B. H. Bay2 1
Department of Surgery, University of Malaya Medical Centre, West Malaysia; 2Department of Anatomy, National University of Singapore, Singapore
Summary The aetiology of the keloid scar has not been completely elucidated. Numerous hypotheses have been proposed in the past to explain the unusual characteristics of the keloid scar. While we do know that there is excessive and ongoing collagen-deposition, the exact triggering stimulus is a subject of conjecture. We present some of our photographic records of keloids and electron microscopic findings of keloid edges and reiterate the sebum hypothesis. We also attempt to explain the features of keloids in the light of the present knowledge of immunology and cell biology. & 2002 Published by Elsevier Science Ltd
INTRODUCTION The keloid scar continues to be a complex and poorly understood subject. The main problem faced by researchers is the lack of an animal model, as keloids affect only humans. At present, evidence from researchers show that transforming growth factor-beta (TGFb), is one of the main cytokines responsible for stimulating the synthesis of matrix proteins, including collagen and fibronectin by fibroblasts (1±3), while suppressing proteases (4). TGFb is released in its latent form by platelets, lymphocytes, macrophages, endothelial cells and fibroblasts (5,6), and has been co-localized with type I and VI collagen at the expanding edge of keloid tissue (7). The most important question remains unanswered. What is the triggering stimulus for the cascade of events in the formation of excessive collagen and extracellular matrix in keloids? A stimulus that is either constant or resurfaces every so often to cause the keloid to progress and enlarge, must be responsible.
Received 11 January 2001 Accepted 25 May 2001 Correspondence to: Dr Eileen Fong MBBS FRCS(Edin), Department of Surgery, University of Malaya Medical Centre, Pantai Valley, Kuala Lumpur 59100, West Malaysia. Fax: 603 7955 7740; E-mail: [email protected]
264
A number of hypotheses have been proposed in the past to explain the keloid phenomenon. Mowlem (8) first suggesting that keratin from either the hair follicle or sebaceous gland could be the initiating stimulus. Yagi et al. (9), observing the absence of keloid in areas devoid of sebaceous glands, proposed sebum to be the responsible factor. They attempted desensitization of known keloid formers with intradermal injection of sebum prior to keloid excision. However, their results were hampered by poor follow-up. Crockett (10) and Peacock et al. (11) believed skin tension to be the stimulus for the excessive production of collagen, citing sites of keloid predilection in areas of so-called tension on the chest, deltoid and back. Skin tension was believed to be the reason why keloid occurred in young people and was almost absent in the elderly. This led Harvey-Kemble (12) to propose intralesional excision of keloids, leaving a rim behind to serve as a splint to counter tension. Hunt et al. (13) considered the hypoxic nature of wounds to be the stimulus for fibroblasts to produce collagen. They noted elevated lactate dehydrogenase enzyme levels in extracellular fluid of wounds associated with anaerobic metabolism. Kischer et al. (14) observed most microvessels in hypertrophic scans and keloids to be occluded, owing to an excess of endothelial cells that bulged into the lumen. They attributed the occlusion to either endothelial cell proliferation or perivascular myofibroblast contraction.
Keloids
That the keloid fibroblast is a mutant monoclonal strain with abnormal increase in collagen synthesis was proposed by Cohen et al. (15) and Abergel et al. (16). However, Russel et al. (17) and Ala-Kokka et al. (18), using radioactive-labeled proline to trace collagen biosynthesis, found that keloid fibroblast produced similar quantities of collagen relative to normal controls. Moreover, Fleischmajer et al. (19) showed that normal human skin fibroblasts are a heterogeneous population of cells which can exhibit up to 20-fold differences in their collagen synthesis, and this heterogeneity is conserved over multiple population doublings. Cohen et al. (20) also proposed that histamine may have a stimulatory effect on collagen synthesis in keloid and hypertrophic scars, noting the histamine content of keloid tissue to be greater than that of hypertrophic scars. Russell et al. (21) noted that histamine caused a marked increase in growth of cultured fibroblast from both normal and keloid tissue, with no difference in growth rate. Mast cells in keloids, responsible for histamine production by degranulation, have been noted by researchers (22,23). In addition to having a stimulatory effect on fibroblast collagen synthesis, Marks et al. (24) also noted the mitogenic effect on microvascular endothelial cells. Among the proponents of the immune-response hypothesis, Oluwasanmi (25) observed an increase in the number of plasma cells and immunoglobulin G deposition along keloid collagen fibres, and suggested that individuals with high levels of circulating immunoglobulin (Ig) are more likely to produce keloids when traumatized. Cohen et al. (26) noted tissue IgG in keloids to be raised compared to normal skin and scar, suggesting a local immune response. No significant difference in serum levels of IgG and no correlation between HLA phenotype and keloid formation was noted. Kischer et al. (27) noted increased levels in all IgG, IgA and IgM in hypertrophic scar and keloid compared to normal skin. No one Ig was significantly raised over the others, suggesting leakage of plasma proteins from microvasculature, possibly due to histamine. CLINICAL EVIDENCE Over the years, we have kept a photographic record of keloids. Despite their variable features, they exhibit a unifying characteristic which prompts us to reiterate the sebum hypothesis. Figure 1 shows a typical earlobe keloid developing as a result of trauma from piercing. What is unusual is the fact that the other ear-hole created in close proximity and at the same time has healed normally with an epithelialized tract. In the other ear of the same patient (not shown), both earholes have healed in the normal manner as well. This puts into question the long-held belief of regional & 2002 Published by Elsevier Science Ltd
265
Fig. 1 Upper ear-hole has formed a keloid while lower ear-hole has healed normally.
Fig. 2 BCG scar at 12 years-of-age forming a keloid. BCG scar at neonate (arrow) heals normally.
susceptibility in keloids, the phenomenon which is often attributed to differences in skin tension. Figure 2 shows a young lady in her thirties who has formed a keloid on her left deltoid from a BCG vaccination given at age 12. Interestingly, a previous BCG vaccination given at neonate, just above the present keloid, has healed normally (see arrow). She also has `spontaneous keloids' on her chest, presumably from previous forgotten incidences of acne (not shown). We have encountered several other examples of this phenomenon ± where the BCG scar at neonate is normal while that given at 12 years-of-age (according to national protocol) has developed into a keloid. Medical Hypotheses (2002) 58(4), 264±269
266 Fong and Bay
Fig. 4 Normal thyroidectomy scar. Keloid formation from puncture site for drainage tube.
Fig. 3 Keloid formation following carbon dioxide laser to tattoo on left deltoid.
Figure 3 shows a young man in his twenties who has extensive tattoo covering his deltoid, chest and back regions. He has had carbon dioxide laser therapy to the tattoo on his left deltoid by another surgeon in an attempt to remove the tattoo marks. Of interest is the fact that he has developed a progressively enlarging keloid on his left deltoid from the carbon dioxide laser, while none of his tattoo-inflicted wounds have formed a keloid. This would seem to add another piece of evidence in support of the sebaceous gland as the responsible stimulus, and not keratin from keratinocytes or hair follicles, as some theories propose. It is a commonly accepted observation that keloids are rare at extremes of life and uncommon before puberty. Piecing the evidence together, it would seem that the occurrence of keloids is an affair dependent on some structure in the skin which is regulated by the sex hormone. The sebaceous gland is a likely candidate. Activity and size of the sebaceous gland is age-related. Sebum excretion increases at puberty and gradually decreases in adulthood after menopause. The active androgen at tissue level responsible for stimulating the sebaceous gland is 5-alpha-dihydrotestoterone, the active metabolite of testosterone and androstenedione (28). Both circulating hormone levels increase at puberty at a time when keloid makes its debut. While the pilosebaceous gland is ubiquitous in its distribution in the body (apart from the palm and sole), it differs in characteristics in different regions of the body Medical Hypotheses (2002) 58(4), 264±269
(29). Vellus follicles are smaller with tiny hairs and are mainly found on the face. The sebaceous follicles have large multilobular glands with thin filmsy hair and are found in acne-prone areas on the face, chest and upper back. The terminal beard follicle has stiff, thick hair and smaller sebaceous gland. The distribution of the oftenseen and well-described phenomenon of `spontaneous keloids' on the back, chest, shoulder and face again suggests the sebaceous gland. These spontaneous keloids have often been thought to occur from previously forgotten incidences of acne ± a condition associated with overactive sebaceous glands. Figure 4 shows a lady with a thyroidectomy scar which has healed in an interesting manner. While the scar on her neck has healed normally, the scar on her chest following a single puncture for the drainage tube has formed a progressively enlarging keloid, which is growing downwards towards her chest. Again, the keloid scar has formed in an area where the sebaceous follicles are perhaps more numerous and voluminous.
DISCUSSION Martin and Muir's (30) work on the role of lymphocytes in wound healing showed a much greater concentration of lymphocytes in keloids as opposed to hypertrophic scars and normal scars. The lymphocytes were persistent in keloids as old as 9 years, suggesting a continuous stimulus. In normal scars, lymphocytes were seen until about 4±5 weeks, and then disappeared rapidly. Also notable is the absence of B-lymphocytes. A similar finding was noted by Appleton et al. (31). They noted numerous apoptotic cells at the interface between dermis and keloid ± a feature characteristic of cell-mediated immune attack. In addition, other nonlymphoid inflammatory cells were seen. & 2002 Published by Elsevier Science Ltd
Keloids
Proposal of a delayed-type hypersensitivity reaction It would seem likely that the chain of events starts with the initial disruption to the pilosebaceous structure, causing leakage of sebum. This antigenic stimulus in sensitive individuals triggers T-lymphocyte recognition and proliferation of antigen-specific T-lymphocytes similar to a delayed-type hypersensitivity reaction (32). The result is clonal expansion of T-lymphocytes migrating to the site of stimulus. Activated T-lymphocytes secrete TGFb, in addition to other cytokines which stimulate the inflammatory response. These cytokines act on macrophages, which serve a phagocytic function, inflammatory leucocytes, which are present at the early stage and mast cells, which secrete histamine and heparin causing features of pruritis and microvascular endothelial cell proliferation (Fig. 5). Some of the progeny develop into antigen-specific memory T cells, which initiate larger secondary immune responses upon subsequent exposure to the antigen. Because the enlarging keloid edge entraps and disrupts adjacent pilosebaceous units, each encounter with the antigen triggers the cascade of events, causing the periodic features of activity and pruritis in keloids. Hence the occurrence of the earlobe keloid from a single prick is a chance phenomenon, depending on the probability of dispersing the antigen. The sebum hypothesis explains the distribution and behavior of keloids. Keloids affect only humans, as animals do not have comparable sebaceous glands. They do not occur on the palms and soles since these areas are devoid of sebaceous glands. Keloids occur in adolescence and early adulthood when sebum production is maximal. Corticosteroids injected into keloids suppress the cellmediated immune response to antigenic stimulus, accounting for the apparent benefit. Repeated injections
Fig. 5 Delayed-type hypersensitivity reaction.
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are required for suppression. Superficial X-ray therapy may confer some benefit due to its known sebumreducing effects (33). Electron microscopy of keloid edges To this end, we have examined the keloid edge in detail. Electron microscopy of a number of active keloid edges show the presence of sebaceous glands. To attempt to look for disruption of the gland right at the invading edge is akin to looking for a split-second event in time. Figure 6 is an electron microscopy of a keloid edge, with whorls of collagen fibres and adjacent sebocyte from sebaceous gland nearby. It would seem highly likely for the enlarging mass of collagen fibres to encroach on to the sebocyte and disrupt it, dispersing sebum into the surrounding tissue and thereby triggering the cascade of events leading to keloid formation. In addition, macrophages are seen with secondary lysosomes effecting phagocytic function (Fig. 7) and mast cells are evident in significant numbers (Fig. 8) in all the active keloid edges examined. Cellular immunology The major type of T cell-mediated immune response in the skin is the delayed-type hypersensitivity reaction. T helper lymphocytes recognize only short sequences of peptide antigens attached to proteins that are encoded in the
Fig. 6 Electron microscopy of keloid edge with adjacent sebocyte. Medical Hypotheses (2002) 58(4), 264±269
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Fig. 7 Macrophages at keloid edge effecting their phagocytic function.
According to Burnet's Clonal Selection Hypothesis, the development of antigen-specific clones of lymphocytes occurs prior to and independent of exposure to antigen (35). The lymphocyte repertoire is extremely large and the mechanism of T lymphocyte loss of self-tolerance in specific clones remains a subject of conjecture. It has been suggested that the loss of self-tolerance may result from abnormal selection of self-reactive clones to T lymphocytes (32). In normal circumstances, these self-reactive clones of immature T cells are negatively selected or deleted in the thymus. Although the thymus involutes after puberty, T lymphocytes continue to arise from bone marrow stem cells in adults and self-reactive clones continue to be negatively selected, since normal individuals remain selftolerant throughout their lives. A possible mechanism for this self-tolerance or T cell clonal anergy is co-stimulator deficiency of the antigen-presenting cells. In situations of tissue injury and inflammation or microbial infection, resident antigen-presenting cells are activated, leading to an increased expression of co-stimulators, loss of selftolerance and local autoimmune reactions. Another possible explanation for this loss of self-tolerance in tissue injury is the release of anatomically sequestered antigens from their normally concealed locations. Known clinical examples of this is the phenomena of post-traumatic uveitis and orchitis following vasectomy (36,37). CONCLUSION The sebum hypothesis, or perhaps more appropriately the sebocyte hypothesis, explains the behavior of keloids. The sebocyte as a self-antigen may result from abnormal selection of self-reactive clones of T lymphocytes, abnormal stimulation of T lymphocytes that are normally anergic to self-antigens or release of self-antigens that are normally inaccessible to the immune system. The keloid microenvironment is heterogenous and the influence of various factors are spatially related. Our evidences suggest a delayed-type hypersensitivity reaction, with the sebum or sebocyte being the persistent antigenic stimulus.
Fig. 8 Mast cells (single arrow) and lymphocyte (double arrows) at keloid edge.
major histocompatibility complex and expressed on the surface of antigen-presenting cells (32). Although the major component of sebum is lipid, sebocytes discharge by a holocrine mechanism, with entire cell disruption (34). It is possible that sebum constituents bind to and modify self-proteins within the sebocyte. Medical Hypotheses (2002) 58(4), 264±269
ACKNOWLEDGEMENTS We would like to thank Mr John A. Kompa, Lecturer in Multimedia Design, Temasek Polytechnic School of Design, Singapore, for his help with the illustration.
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