Immunologic marker studies of the lymphoid leukemias have greatly improved the precision of diagnosis of these disorders by providing specific information regarding the lineage and stage of maturation of the malignant cells. Such studies have also enhanced our understanding of normal lymphocyte development, permitting reproducible identification of lymphoid cells in discrete developmental stages. By elucidating the functions of lymphoid cell differentiation antigens, it has been possible to gain insight into the signal transduction mechanisms by which these cells interact among themselves and with other cell types. Similar studies have shown that ALL is an immunophenotypically heterogeneous disease with clinically important subtypes representing clonal expansions of lymphoblasts at different stages of maturation. Furthermore, successful correlation of immunophenotype with certain karyotypic and molecular abnormalities, which appear to underlie most or all leukemias, were made possible by the inclusion of immunologic marker assessment. Interestingly, many of these phenotype-related abnormalities have involved either the Ig or TCR genes, thus providing additional clues to the mechanisms of leukemogenesis. Knowledge of the immunologic features of leukemic cells has been essential for the generation of phenotype-specific response data in the context of modern therapy for ALL. With wider use of intensive treatment, the traditional prognostic distinctions among immunophenotypes have begun to disappear; however, certain classes of agents have more favorable toxicity/efficacy ratios against some immunophenotypes than others, justifying continued efforts to target therapy by immunologic species of ALL. Antibody-toxin conjugates, or immunotoxins, have induced complete responses in preliminary trials and may prove clinically useful, perhaps in combination with chemotherapy, if their toxic side effects can be controlled. Finally, immunologic markers may serve as sensitive targets for the detection of minimal residual disease; the clinical usefulness of this approach will depend on prospective comparisons with molecular methods.

Programmed cell death (apoptosis) is a normal process by which cells are eliminated during normal embryonic development and in adult life. Disruption of this normal process resulting in illegitimate cell survival can cause developmental abnormalities and facilitate cancer development. Normal cells require certain viability factors and undergo programmed cell death when these factors are withdrawn. The viability factors are required throughout the differentiation process from immature to mature cells. Although many viability factors are also growth factors, viability and growth are separately regulated. Viability factors can have clinical value in decreasing the loss of normal cells including the loss that occurs after irradiation, exposure to other cytotoxic agents or virus infection including AIDS. There is no evidence that occurs after irradiation, exposure to other cytotoxic agents or virus infection including AIDS. There is no evidence that cancer cells are immortal. Programmed cell death can be induced in leukemic cells by removal of viability factors, by cytotoxic therapeutic agents, or by the tumor-suppressor gene wild-type p53. All these forms of induction of programmed cell death in leukemic cells can be suppressed by the same viability factors that suppress programmed cell death in normal cells. A tumor-promoting phorbol ester can also suppress this death program. The induction of programmed cell death can be enhanced by deregulated expression of the gene c-myc and suppressed by the gene bcl-2. Mutant p53 and bcl-2 suppress the enhancing effect on cell death of deregulated c-myc, and thus allow induction of cell proliferation and inhibition of differentiation which are other functions of deregulated c-myc. The suppression of cell death by mutant p53 and bcl-2 increases the probability of developing cancer. The suppression of programmed cell death in cancer cells by viability factors suggests that decreasing the level of these factors may increase the effectiveness of cytotoxic cancer therapy. Treatments that downregulate the expression or activity of mutant p53 and bcl-2 in cancer cells should also be useful for therapy.

Three distinct clinical syndromes occur in patients with increased numbers of circulating LGL. Patients with T-LGL leukemia have clonal proliferations of CD3+ LGL typically associated with chronic neutropenia and autoimmune features. NK-LGL leukemia is characterized by clonal CD3- LGL proliferation with an acute clinical presentation marked by massive hepatosplenomegaly and systemic illness. However, most patients with increased numbers of CD3- LGL do not have clinical features of NK-LGL leukemia and have a chronic clinical course. X-linked gene analyses have supported a polyclonal LGL lymphocytosis in this syndrome. Further studies are needed to determine whether clonal progression can occur in these patients.

Recent developments have contributed important information to understanding the role of spectrins in the RBC membrane skeleton and nonerythroid cells. Many questions can now be framed, informed by structural knowledge of various spectrin subunit types and alternatively spliced variants, that previously could not have been addressed. Their solution in the coming years will likely lead to further advances with direct relevance to biology and medicine.

Available evidence indicates that qualitative changes in hematopoietic stem cells and progenitors, such as the decision of stem cells to self-renew or differentiate, or selection of lineage potentials by the multipotential progenitors during differentiation (commitment), are intrinsic properties of the progenitors and are stochastic in nature. In-contrast, proliferative kinetics of the progenitors, namely survival and expansion of the progenitors, appear to be controlled by a number of interacting cytokines. While proliferation and maturation of committed progenitors is controlled by late-acting lineage-specific factors such as Ep, M-CSF, G-CSF, and IL-5, progenitors at earlier stages of development are controlled by a group of several overlapping cytokines. IL-3, GM-CSF, and IL-4 regulate proliferation of multipotential progenitors only after they exit from G0 and begin active cell proliferation. Triggering of cycling by dormant primitive progenitors and maintenance of B-cell potential of the primitive progenitors appears to require interactions of early acting cytokines including IL-6, G-CSF, IL-11, IL-12, LIF, and SF. Currently, this simple model fits our understanding of the interactions of growth factors with hematopoietic progenitors. Naturally the model risks oversimplification of a very complex process. However, because the model is testable, it will hopefully challenge investigators to design new experiments to examine its validity.

There have been several new developments in the field of autoimmune neutropenia over the past decade. Neutropenia caused by antibodies directed against granulocyte precursor cells, the oligoclonal nature of antineutrophil antibodies, and the expanding knowledge of neutrophil antigens, particularly in relationship to autoantibodies, are exciting new areas of investigation. Knowledge has also been advanced in the effector mechanisms of neutrophil autoantibodies and the effect of autoantibodies on the neutrophil function. In addition, some clinical syndromes of immune neutropenia have been better defined over the past decade, such as autoimmune neutropenia of infancy and chronic idiopathic neutropenia in adults. The past decade also saw interesting developments in the treatment of immune neutropenia, particularly in the use of gammaglobulin preparations and more recently in the advent of hematopoietic growth factors. This review focuses on these newer aspects of autoimmune neutropenia.