Editorial:
"Hodgkin's Disease -- Time for a Change".
Robert S. Schwartz, M.D.
Reed-Sternberg cells and their variants have long held fascination for students of Hodgkin's disease. What are these cells, and how do they relate to the clinical manifestations of the lymphoma? Recent work indicates that they are clones of neoplastic B cells that, by secreting potent cytokines, not only cause the symptoms of Hodgkin's disease but also promote their own growth and evade immune surveillance (1).
The morphologically distinctive and multinucleated Reed-Sternberg cells -- classic Reed-Sternberg cells -- may derive from T cells in some cases, but the weight of the evidence suggests that they usually originate from B lymphocytes in germinal centers of lymphoid tissue. This view has been corroborated and expanded by methods of deciphering immunoglobulin variable-region (V) genes. During B-cell development, particular V genes join together in ways unique to each B cell. These rearrangements occur exclusively in B cells, and the nucleotide sequence of the rearranged genes is a molecular fingerprint of that cell and its progeny (2).
Thanks to the polymerase chain reaction (PCR), it is possible to determine the nucleotide sequences of the rearranged immunoglobulin genes from a single B cell. In a histologic section of tissue affected by Hodgkin's disease, one can identify Reed-Sternberg cells morphologically and by surface markers, pluck them out individually, and sequence the rearranged V genes in each cell. Rajewsky's group, which pioneered this method, has applied it to 14 cases of Hodgkin's disease, mainly of the mixed-cellularity and nodular-sclerosis types (3, 4). In 13 of these cases, all the Reed-Sternberg cells from a given biopsy specimen had the same or a very similar immunoglobulin-gene fingerprint. They were, therefore, B cells derived from a single clone. These clones can disappear from sight after treatment only to return years later, disseminated throughout the body and bearing the molecular fingerprint of the Reed-Sternberg cells in the original biopsy specimen -- in short, they behave like malignant cells (5). The findings of the Rajewsky group have been substantiated with other genetic markers (6, 7), but some investigators have reported Reed-Sternberg cells with unrelated V genes in the same biopsy specimen (8). These discordant findings may indicate a polyclonal proliferation of Reed-Sternberg cells or only contamination of the PCR. Technical faults lurk in the shadows of these exacting methods, and an anomalous result is always worrisome.
Two papers in this issue of the Journal report analyses of the immunoglobulin genes of individual lymphocytic and histiocytic (L&H) cells from patients with nodular lymphocyte-predominant Hodgkin's disease, a distinctive and rare form of the disorder in which cells with unusual folded and lobate nuclei (L&H cells) lie against a dense background of small lymphocytes and histiocytes. In all 16 cases of nodular lymphocyte-predominant Hodgkin's disease (11 studied by Marafioti et al. (9) and 5 studied by Ohno et al. (10)), L&H cells with the molecular features of a monoclonal population of B cells were found. In a few cases, members of the same clone were found in different nodes removed either simultaneously or sequentially from the same patient. In one of Ohno's specimens, only 2 of 14 L&H cells had the same V gene sequences (10), again raising the possibility of polyclonality or technical error. Marafioti et al. (9) suggest that their results, showing differences in V gene alterations between L&H cells and classic Reed-Sternberg cells, add to the evidence that lymphocyte-predominant and classic Hodgkin's disease are distinct entities. Whether this alone is enough to distinguish the two disorders is uncertain, but reclassifying lymphocyte-predominant Hodgkin's disease as a non-Hodgkin's B-cell lymphoma could bring it into better focus. Clinically, the two disorders clearly differ. The lymphocyte-predominant type is indolent and has an excellent prognosis, whereas classic Hodgkin's disease takes a more aggressive course.
Kanzler et al. (4), who first convincingly showed that classic Reed-Sternberg cells are monoclonal B cells, also found numerous V gene mutations in them. Moreover, stop codons and other alterations that disable the gene were common. These genetic changes also occur in normal B cells during the immune response to an antigen in the germinal center of a lymph node (11). After the freshly formed B cell arrives in a lymphoid follicle, its survival depends on signals from an antigen and from helper (CD4+) T cells. These stimuli not only save the B cell from apoptotic death, but also cause it to mutate the V genes it had rearranged while it was differentiating in the bone marrow. Normally, the V genes of antigen-stimulated B cells undergo only a limited number of mutations -- an advantage, because excessive mutation can cripple the genes (4). The biologic effect of mutation of a V gene is to alter the affinity of the B cell for the immunizing antigen by replacing amino acids in the antigen-binding site of the immunoglobulin receptor. B cells with high-affinity receptors survive, whereas B cells with low-affinity receptors, or disabled V genes, die. This is important because flawed V genes -- which should prove lethal to normal B cells -- have been found in Reed-Sternberg cells (4). What keeps the Reed-Sternberg cells alive, despite useless V genes? Kanzler et al. (4) propose that Epstein-Barr virus (EBV) rescues it, pointing to a study showing EBV infection of Reed-Sternberg cells in at least half of cases of Hodgkin's disease (12). By contrast, L&H cells consistently test negative for EBV and only a few have disabling V gene mutations (9).
Reed-Sternberg cells may harbor EBV, but they do not behave like EBV-transformed B cells or any normal B cell. The unusual functional properties of Reed-Sternberg cells may hold the answer to a puzzle. Of the cells in tissue affected by Hodgkin's disease, only about 1 percent are Reed-Sternberg cells. The rest are lymphocytes, macrophages, granulocytes, and often eosinophils. How can so few Reed-Sternberg cells provoke the symptoms and signs of Hodgkin's disease? It is likely that potent cytokines enable them to broadcast biologic effects widely. Reed-Sternberg cells produce at least 12 cytokines, including interleukin-1, interleukin-6, and tumor necrosis factor, any of which could account for the constitutional symptoms of Hodgkin's disease (1). CD4+ T cells, abundant in Hodgkin's tissue, become activated through adhesion proteins and cytokines of Reed-Sternberg cells. They reciprocally stimulate Reed-Sternberg cells, which respond with an unphysiologic profile of activating and inhibitory cytokines. Aberrant production or dominance of one or more cytokines could explain aspects of the variants of Hodgkin's disease: eosinophilia in mixed cellularity (interleukin-5), fibrosis in nodular sclerosis (transforming growth factor (beta) and platelet-derived growth factor), and lymphocytopenia in the lymphocyte-depletion form (tumor necrosis factor and transforming growth factor (beta)). Whether L&H cells have the overt functional abnormalities of classic Reed-Sternberg cells is unknown but doubtful because, in contrast to classic Hodgkin's disease, lymphocyte-predominant Hodgkin's disease is almost always asymptomatic and slow-growing.
Many complexities of Hodgkin's disease remain, but it is now clear that Reed-Sternberg cells and L&H cells usually arise in the germinal center from a clone of antigen-stimulated B cells. But with this similarity, the connection between classic Hodgkin's disease and lymphocyte-predominant Hodgkin's disease ends. We know that Reed-Sternberg cells and L&H cells retain the ability of normal B cells to mutate V genes, but Reed-Sternberg cells can acquire too many V gene alterations -- enough to kill a normal B cell. Other genetic changes, currently unknown, must allow Reed-Sternberg cells to avoid death, achieve immortality, and acquire malignant properties. The identification of these mutations and what causes them is a major challenge to investigators of Hodgkin's disease.
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