In June 1995, a workshop held at the Centers for Disease Control and Prevention (CDC) attracted a number of physicians, epidemiologists, and laboratory scientists with special interest in cancers of the immune system. They gathered to discuss laboratory findings from studies that the Agency for Toxic Substances and Disease Registry (ATSDR) conducted to evaluate the impact of environmental exposures on human health. Of the several thousand human blood samples tested over a 4-year period, 11 revealed striking changes in their B-cells, a central component of the immune system. These changes included an increase in the proportion of B-cells among lymphocytes, as well as cell surface changes usually associated with the malignant B-cells of chronic lymphocytic leukemia or low grade non-Hodgkin's lymphoma.
 
    This proceedings volume contains a varied collection of material from the workshop: findings from the ATSDR studies, reports on B-cells and the malignancies that arise from them, and background on the epidemiology of envi-ronmental risk factors for such B-cell malignancies. The material and especially the nomenclature can be quite confusing even for experts, let alone
for lay persons. Therefore, in this preface, we will attempt to provide enough background so that the interested reader can access the information that follows.

    Lymphocytes are the central recognition cells of the immune response; that is, they are the cells that identify foreign material (like bacteria) as invaders.  When a single lymphocyte has been activated, it can multiply into many daughter cells, and the family of cells derived from the original lymphocyte is called a clone. The immune system of a normal person contains millions of
different clones. During an immune response to an invader or a vaccine, hundreds or thousands of clones will expand, which is called a polyclonal proliferation.

    Malignant (cancerous) lymphocytes, like other cancer cells, lose control over their multiplication, and their continued expansion eventually causes clinical disease. Because all malignant lymphocytes arise from a common precursor, they are all members of single clone, a so-called monoclonal prolifera-tion.  The two malignant diseases mentioned above, chronic lymphocytic leukemia and non-Hodgkins' lymphoma, are caused by the uncontrolled proliferation and accumulation of monoclonal lymphocytes.

    Lymphocytes are usually easy to identify by their microscopic appearance, and malignant lymphocytes have been recognized as the cause of chronic lymphocytic leukemia and non-Hodgkins' lymphoma for many decades. How ever, the two major types of lymphocytes, B-cells and T-cells, cannot be distinguished merely by appearance. Over the last 15 years, advances in a technique called flow cytometry have made their distinction possible by identifying
unique receptor molecules on their cell membranes. The receptors are commonly called cell surface markers or cell surface antigens, and the pres-ence or absence of such markers is called a lymphocyte phenotype (LPT).  Most of the findings presented in this volume were made by using flow cytometry to identify LPT, and the discussion in Chapter 2 gives more details about
it. More recently, genetic analysis has been used to identify B-cells and T-cells, and the two techniques are now considered routine and complementary.
 
    LPT and genetic analyses have been part of the routine work-up for chronic lymphocytic leukemia and non-Hodgkins' lymphoma for only a comparatively short time, and the bulk of epidemiologic data compiled by the surveillance program of the National Cancer Institute does not include the results of these analyses.  Nonetheless, individual investigations have made it clear that almost all cases of chronic lymphocytic leukemia and non-Hodgkins' lymphoma in the western hemisphere are caused by malignant expansions of B-cells rather than T-cells. The term “B-CLL” is used to specify the B-cell origin of chronic lymphocytic leukemia; similarly, “B-NHL” is used to specify the B-cell origin of non-Hodgkins' lymphoma. Strictly speaking, a B-cell origin should not be specified without specific documentation from LPT or genetic
analysis. However, because T-cell malignancies are so rare in the Western hemisphere, the terms specifying the B-cell origin of these diseases are often used interchangeably with the more general terms for the diseases. In this volume, we have tried to refer to B-CLL and B-NHL only when justified by laboratory evidence, and to use the more general terms CLL and NHL when
referring to the mostly unspecified epidemiologic data.

    Because B-CLL and B-NHL are caused by malignant expansions of monoclonal B-cells, the determination of monoclonality is critical to diagnosis. This is particularly true in the early stages of disease, when the number of malig-nant cells has not yet become overwhelming. B-cell monoclonality may be documented either by LPT or genetic analysis, as described in several papers in the Immunobiology section of this volume.  However, the presence of a small number of monoclonal B-cells does not in itself define a disease.
 
    Chronic lymphocytic leukemia develops slowly, and it is often discovered only when laboratory tests reveal an increased number of lymphocyte in the blood. In normal adults, peripheral blood lymphocyte counts range from about 1500 to 4000 lymphocytes per microliter. To diagnose chronic lymphocyte leukemia, counts must reach at least 10,000 lymphocytes per microliter.
However, if there is evidence of monoclonality, chronic lymphocytic leukemia can be diagnosed with lymphocyte counts as low as 5,000 cells per microliter.  The routine use of automated blood cell counters has increased the detection of asymptomatic elevated lymphocyte counts, which are best evaluated by follow-up tests using flow cytometry. Just as automated blood cell counters are able to detect early increases in lymphocyte counts, flow cytometry can readily detect the presence of an emerging B-cell clone far below the level of 5000 cells/microliter. One of the central issues discussed in this volume is the rela-tion between the laboratory finding of low-level B-cell monoclonal lymphocy-tosis and clinical diagnosis of chronic lymphocytic leukemia.

    Since 1991, ATSDR has conducted laboratory testing on about 6000 indi-viduals living close to hazardous waste sites and on demographically-matched control populations. Part of the immune biomarker analysis consisted of a lymphocyte subset analysis performed at CDC. In the first year of these studies, one individual was noted to have an abnormal B-cell phenotype (see
Chapter 2) suggestive of the increased B-cells seen in B-CLL. Shortly after this observation, the phenotype marker CD5 was added to the immune test panel, in part because of its characteristic pattern in B-CLL. By 1993, five more indi-viduals with increased numbers of B-cells and abnormal flow cytometric patterns were seen; these findings were collectively termed “B-CLL-like.” In 1994, a study at one site revealed 5 individuals with B-CLL-like patterns,
which prompted ATSDR to request analysis for monoclonality. As expected, these analyses (performed at FDA) confirmed the presence of a B-cell mono-clonal lymphocytosis (BCML) in each of the three individuals tested. This strongly suggests (but does not prove) that the other eight individuals also had BCML.

    Although B-CLL is the most common form of leukemia in the United States, the number of people with these B-CLL-like findings was larger than we would have suspected on the basis of our clinical experience and current knowledge of the incidence and prevalence of B-CLL. The epidemiologic analysis of these findings is one of the major discussions of this volume. Suffice it
to say, we found what appeared to be an unexpected number of individuals with a B-CLL-like immunophenotype, prompting the workshop held in June 1995. One of the most important recommendations to come out of this work-shop was that these findings must be confirmed, and that individuals with a B-CLL-like immunophenotype need to be as fully characterized as possible.

    At this time, data from the ATSDR studies cannot support or rule out environmental exposures as a cause of BCML. The B-CLL-like cases were almost twice as common among target subjects living proximate to hazardous waste sites as in the comparison groups. However, the small number of observations, the uncertain exposure status of individuals in the target populations, and the unknown prevalence of BCML in a normal population make any
definitive conclusions impossible at this time. Even if an environmental association is weak or absent, the high prevalence of BCML detected in the ATSDR studies will have a major impact on our understanding of B-CLL and projec-tions for the health of an aging population.

    We have made every attempt to see that this proceedings volume reflects accurately the state of our knowledge concerning BCML, its possible relationship to environmental exposures, and its relevance to B-CLL. In reviewing this volume, please note first the Table of Contents. After the “Executive Summary” and “Workshop Findings, Recommendations and Agency Responses,”
there are three major sections devoted to original reports from the workshop participants. The first section (“Findings From ATSDR Health Studies”) contains the major findings that formed the basis of the workshop. The second section (“Immunobiology and Molecular Genetics of B-Cell Lymphoproliferative Disorders”) discusses several aspects of B-cell biology that have a direct
bearing on how B-CLL is presently understood. The third section (“Risk Factors and Epidemiologic Considerations for B-cell Lymphoproliferative Disorders”) primarily is focused on what we know about the epidemiology of B-CLL and it should be further characterized with respect to environmental risk factors.

     For the final section of the volume, Professor RA Cartwright has written a summary chapter of the entire workshop. Although he was unable to attend the workshop in person, Professor Cartwright was highly intrigued by the findings and has remained in communication with the editors. We are honored to have his expertise represented as the conclusion of this volume.

    We hope that this document draws attention at the highest levels within the Public Health Service, and we intend to publish it electronically on CD and/or on a Web site in the near future. We further hope that it will help to secure support for conducting the recommended studies and for organizing a follow-up workshop that includes an inter-laboratory comparison study. To
date, the convening of the workshop and editing of this volume has been and continues to be a rewarding experience. We gratefully acknowledge support of all the authors and participants and encourage their continued support in this ongoing endeavor. We are especially grateful to Barbara Lord for her diligence, enthusiasm, and skill in preparing this publication.