Etiology and Pathogenesis
July 25, 1994
As the epidemiology suggests, many researchers have tested the hypothesis of several different etiologic agents with the hopes of finding one that is associated with KS. As sexually transmitted agents have been associated with other malignancies, it has been hypothesized that a sexually transmitted agent may be associated with KS.
To date the following viruses have been studied, without evidence for viral sequences in KS cells: HTLV-I, HTLV-II, HIV-1, HBV, HHV-6, EBV, CMV, HSV-1, HSV-2, papova and polyoma viruses (Salahuddin 1988; Marmor 1984).
Cytomegalovirus once seriously considered as a possible cofactor, has been ruled out due to the low prevalence of KS in intravenous drug users and a variety of other studies that could not confirm the that CMV infection was more common in people with KS (Marmor 1984; Salahuddin 1988).
Huang et al. (Huang 1992) suggested that Human Papilloma Virus-16 (HPV-16) may be associated with KS (92). Polymerase chain reaction was used to detect HPV DNA fragments in tumor tissues. HPV DNA fragments were found in KS lesions from 11 of 69 homosexual men with HIV-related KS, in 3 of 11 HIV-negative homosexual men, and in 5 of 17 elderly men and women with classic KS. However, Biggar et al. (Biggar 1992) were unable to detect HPV in KS lesions. Using PCR and in situ hybridization, no evidence of HPV was found in 14 KS tissue samples from 3 homosexual men, 6 elderly people, and five HIV-negative individuals. In addition, Fugiwara et al. (Fugiwara 1993) were unable to detect HPV-16 DNA sequences in HIV-related and classic KS tissue samples.
Researchers continue to search for etiologic agents. However, some feel that there is no one agent responsible for KS and that KS is the end result of a cascade of events resulting from hyperactivation of the immune system. These questions can only be answered through additional pathogenesis research.
Is KS a True Malignancy?
This is an extremely integral question that, when or if it can be answered, can have some serious implications for the future management of this disease. This unanswered question has had implications on more than treatment choice. The inability to classify KS as either a malignancy or an infectious disease has had, and continues to have, implications for the funding and coordination of research efforts. While the National Cancer Institute (NCI) and the National Institute of Allergy and Infectious Diseases (NIAID) fund basic research and clinical studies, neither the NCI nor NIAID have taken the leadership role in the funding and coordination of these efforts. This same lack of leadership and responsibility is missing in treatment. As stated by Susan Krown, an oncologist in New York City, "Just as many infectious disease specialists feel that Kaposi's Sarcoma is a peripheral issue in AIDS, so too many oncologists feel that understanding and developing treatments for Kaposi's Sarcoma is not germane to the central issues of our specialty." (Krown 1993)
More questions can be asked than statements made about this hyperplasia. Markers of true malignancy such as clonality and/or clonal chromosomal abnormalities, have not been demonstrated (Levine 93). In the early stages of disease there is no one dominant cell line. As disease progresses and immunosuppression becomes more severe, the disease becomes more aggressive. One malignant cell line becomes dominant, chromosomal change occurs, and lesions may begin to take on a tumor-like behavior. In addition, the dependence of KS on a variety of cytokines and growth factors also suggests aspects of a true malignancy. Robert Gallo of the Laboratory of Tumor Cell Biology (LTCB) at the NCI suggests that as KS progresses into late stage disease, it begins to resemble a true malignancy (Robert C. Gallo, personal communication).
Robert Gallo's lab has successfully isolated an immortalized cell line, referred to as KS Y- 1. This cell line is able to generate long lasting tumors and metastases in nude and SCID mice. This KS cell line shows the same phenotype in vitro as KS cell strains and can induce angiogenesis in vivo (Lundardi-Iskandar 1993).
Many questions remain regarding the classification of KS into any disease type. If KS is not a cancer than why does it respond to chemotherapy in systemic disease? If KS is a cancer than why does KS spontaneously regress in transplant recipients after immunosuppressive therapy has been stopped? No other cancer responds to the restoration of the immune system (Alvin Friedman-Kien, personal communication). The ability to answer these questions lies in pathogenesis studies. While we have gained a wealth of knowledge regarding the pathogenesis through the ability to culture KS cells in long term tissue cultures, we still do not know the cell of origin.
KS Cell of Origin: The Missing Link
Several studies have been conducted in order to determine the cell of origin of KS. Potential progenitors of the spindle-shaped KS cells are mesenchymal in origin. They have variously been reported to have characteristics of endothelial cells of vascular (Corbeil 1991) and lymphatic origin (Salahuddin 1988), smooth muscle cells (Weich 1991) and dermal dendrocytes (Huang 1993). Overwhelming evidence does not exist for any one specific cell type. Roth et al. suggested that due to the heterogeneity of the cultivated cells, KS tumors may be composed of spindle-shaped cells of various origins (Roth 1992).
The establishment of KS cell cultures in the laboratory has made it possible to study factors associated with the growth of KS cells. Nakamura et al. (Nakamura 1988) have successfully maintained long-term KS cell cultures using growth factors released by CD4 cells infected with retroviruses (HTLV-I, HTLV-II, and HIV-1). It was also noted that factors associated with KS cell growth were also associated with the growth of endothelial cells. However, this was not true for the reverse; factors associated with endothelial cell growth were not associated with KS cell growth. This suggests differing growth requirements for the different cells lines.
After KS cells have initiated cell growth from as yet undetermined factor(s), they have the capacity to sustain their own growth, as well as the growth of normal endothelial cells, fibroblasts and other cell types, and to induce new blood vessel proliferation (Ensoli 1989; Salahuddin 1988). These reports suggest that the growth of KS and other cells is maintained by paracrine and autocrine factors produced by retrovirus- infected CD4 cells and by KS cells themselves. Salahuddin et al. supported these findings by demonstrating the induction of KS cell proliferation in nude mice. Five days after being inoculated with cultured human AIDS-KS-derived cells, nude mice developed a transient angiogenic reaction, and developed lesions of mouse cell origin, histologically similar to human KS, at the site of inoculation.
Several cytokines and growth factors have been associated with the possible initiation and continued growth of KS cells and angiogenesis:
Fibroblast Growth Factors (FGFs) have been associated with angiogenesis. The expression of both FGF and FGF receptors has been demonstrated in cultured KS cells as well as in fresh biopsies (Li 1993). Li et al. reported that these findings suggest that FGF and FGF receptors play a role in the pathogenesis of KS. However, it is unclear whether this is due to their oncogenic and/or angiogenesis properties. These findings also support the paracrine and autocrine mechanisms that contribute to the development and growth of the KS cells. An in vitro study of antisense oligonucleotides against bFGF showed that they can block AIDS-KS cell proliferation in response to inflammatory cytokines produced by activated T-cells. This report suggests that inflammatory cytokines induce KS-cell proliferation through an increase of bFGF activity and that they may promote the expression of bFGF found in KS lesions (Samaniego 1993). Ensoli et al. reported on the use of bFGF antisense in nude mice injected with bFGF. This study showed that bFGF antisense was successful in blocking KS cell growth and inhibited the angiogenic activity of these cells such as the stimulation of normal endothelial cell growth, migration, and invasion. These effects were seen in a dose-dependent manner (Ensoli 1993 b).
Numerous studies have suggested a role for inflammatory cytokines such as IL-1, IL-6 and TNF-alpha, in the pathogenesis of KS (Barillari 1992; Miles 1990; Salahuddin 1988; Nakamura 1988). IL-1 has been implicated in indirectly inducing angiogenesis through working with cells of the immune system. IL-1 is induced in monocytes and macrophages in HIV infected individuals. In addition it has been shown that like bFGF, IL-1 is also produced by KS cells, suggesting the continued proliferation of KS cells and mesenchymal cell types in an autocrine and paracrine fashion (Ensoli 1993 b).
IL-6 as well has been shown to act in an autocrine fashion in KS cell proliferation (Miles 1990). This was demonstrated by the reduction of IL-6 protein translation when an IL-6 antisense oligodeoxynucleotide was added. Proliferation continued when exogenous IL-6 was added. Using involved and uninvolved tissue from a patient with AIDS-KS, Miles et al. showed that IL-6 and IL-6 receptors were in great excess in the involved skin relative to the uninvolved skin. Miles suggests that the mitogenic activity of IL-1 and bFGF for AIDS-KS cells might be by inducing IL-6 production or altering IL-6 responsiveness of these AIDS-KS cells. This may be one reason for the variation in prognosis for patients with AIDS-KS. Since opportunistic infections increase cytokines such as IL-1 and TNF, the expression of these cytokines may be the reason why people with KS have rapid progression with concurrent opportunistic infections. While elevated IL-6 is not unusual in the absence of AIDS-KS, the expression of IL-6 receptors in endothelial cells is of interest in this study.
TNF has also been associated with KS pathogenesis. A study of skin biopsies from 6 patients with AIDS-KS showed that all 6 patients showed increased amounts of IL-6 and TNF in the epidermal cells of the tumor. Neither cytokine was found in the endothelial cells of the tumors. The authors were unsure of the significance of this finding.
A variety of other growth factors have been associated with the proliferation of KS cells through autocrine and paracrine mechanisms, including platelet derived growth factor (PDGF), vascular endothelial growth factor (VEGF) also known as vascular permeability factor (VPF) (Weindel), transforming growth factor (TGF), and granulocyte-macrophage colony stimulating factor (GM-CSF) (Ensoli 1994). TGF is an immunosuppressive cytokine that is released from PBMCs upon infection with HIV and was found to stimulate KS cell growth (Ensoli 1994).
It has been suggested that it is the above growth factors that lead to the angiogenesis involved in KS tumor growth. The question is which one or which combination of growth factors is involved.
While there are several growth factors involved in the proliferation of KS-spindle cells, there has, until recently, been no explanation for its association with HIV infection. Although it was once thought that HIV might play a direct role in causing KS, more recent studies suggest a more indirect role mediated by the HIV tat protein. Male transgenic mice developed KS-like lesions when tat was expressed (Vogel 1988). In addition, tat has been shown to stimulate the growth of KS cells as well as the growth or normal vascular (endothelial) cells after exposure to inflammatory cytokines (Barillari 1992). Farewell et al. suggest that tat mimics bFGF in the presence of activated endothelial cells (Farewell 1993). Ensoli et al. have shown that tat and bFGF act synergistically to induce KS-like lesions in mice, and have suggested a mechanism by which inflammatory cytokines (TNF, IL-1, and gamma-IFN), tat, and bFGF work in cooperation in the pathogenesis of KS (Ensoli 1993). These results suggest it may be useful to reinvestigate tat inhibitors against AIDS-KS.
Etiology + Pathogenesis Recommendations: Further explore the epidemiologic and pathogenetic determinants of AIDS-KS, and define whether or not an infectious agent or cofactor is involved. Further study epidemic KS incidence in HIV-negative gay and bisexual men. Further elucidate and confirm the contribution of inflammatory cytokine pathways to the pathogenesis of KS. Foster new collaborative research projects among epidemiologists, immunologists and KS oncologists. Establish a national tissue and serum repository to provide access to clinical samples from HIV-infected persons at risk for and those developing AIDS-related neoplasms, and guarantee equitable access to extramural researchers to these valuable samples.
This article was provided by Treatment Action Group. It is a part of the publication The KS Project Report: Current Issues in Research & Treatment of Kaposi's Sarcoma.