Edward J. Moticka

T Lymphocyte Subpopulations

Thymus-derived lymphocytes perform a variety of functions in the adaptive immune response. These include activating the inflammatory response, helping B lymphocytes produce antibodies, rejecting foreign grafts, eliminating virally infected cells, and regulating ongoing immune responses. The realization of this dichotomy stimulated a search for functional subpopulations of T lymphocytes based on phenotypic differences. Initially, T lymphocytes were separated into two populations, CD4 + and CD8 + , based on the expression of cell surface molecules that could be detected with monoclonal antibodies. This division was correlated with function (CD4 + lymphocytes are helpers while CD8 + lymphocytes are cytotoxic) and the mechanism used by these lymphocytes to interact with foreign antigens (CD4 + lymphocytes recognize class II histocompatibility molecules while CD8 + lymphocytes recognize class I histocompatibility molecules). Subsequently, subpopulations of CD4 + T lymphocytes have been isolated based on the cytokines that induce their activation as well as the cytokines they synthesize and secrete following activation; this division is correlated with their function. At least five functional subpopulations of CD4 + T lymphocytes are known; manipulation of these populations provides intriguing methods to regulate ongoing adaptive immune responses in several clinical situations such as autoimmune disease, cancer, and allograft rejection.

The Clonal Selection Theory of Antibody Formation

The clonal selection theory of antibody formation is the most important advance in immunology in the past hundred years. Previous models of antibody formation were developed in the absence of essential information concerning the chemical nature of antibodies, the cell types responsible for antibody production, and how the immune system “knew” what specificities needed to be synthesized. In 1900, Paul Ehrlich proposed the side-chain theory of antibody formation. This was criticized due to a lack of knowledge of antibody structure. Until the 1950s, the most logical explanation of antibody formation was that the antigen served as a template to instruct cells to produce complementary binding molecules. However, information about the role of the amino acid sequence of proteins in determining molecular structure and the mechanisms of protein synthesis made instruction models difficult to defend. During the 1950s, several selection models were proposed. Niels Jerne proposed the natural selection hypothesis that moved the field from antigen instruction to antigen selection. Finally, F. Macfarlane Burnet offered a model, the clonal selection theory, that proposed the existence of a large number of antibody-forming cells, each of which is genetically preprogrammed to produce a unique antibody specificity. A similar model was simultaneously proposed by David Talmage. This theory has withstood numerous attempts to disprove it and is now the basis for our current understanding of the immune system.

Chapter 36 – Transplantation Immunology

Physicians have long considered the transplantation of tissue and organs to individuals with various pathologies to be the Holy Grail. Initially they were faced with technical issues such as maintaining viability in organs removed from donors and anastomosing blood vessels. These issues were solved during the first half of the twentieth century, and in 1954, Joseph Murray and colleagues performed the first successful kidney transplant between identical twins. Subsequently the problem of immunologic rejection needed to be addressed. Although transplantation of relatively avascular tissue (cornea) was successful, the transfer of vascularized organs between donors and recipients who were not genetically identical routinely failed. Studies in the early 1900s demonstrated that transplanted tumors expressed cell surface antigens that stimulated adaptive immune responses. In the 1940s and 1950s, Peter Medawar showed that rejection of healthy tissue was due to an immunological reaction and that the antigens responsible for inducing this response were the same as those involved in tumor rejection. Further studies revealed that these antigens are coded for by genes in the major histocompatibility complex. The destructive reaction involves activation of T lymphocytes in the recipient. This knowledge led to the development of several drugs that can control these responses and has made organ transplantation the success it currently is.

Chapter 35 – Lymphoproliferative Diseases

Lymphocytes undergo malignant transformation and form tumors as do other somatic cells. The genetic events leading to cancer affect lymphocytes at all stages of differentiation from embryonic stem cells to fully mature antibody and cytokine secretors. Lymphoid tumors are classified as lymphomas, lymphocytic leukemias, or plasma cell dyscrasias based on criteria including anatomical location of the tumors, maturational stage of the lymphocyte, and presence or absence of secretory products. Each of these classifications comprises several discrete clinical diseases with unique patterns of genetic abnormalities. While all of these cancers have existed in the population for hundreds, if not thousands, of years, the initial pathological descriptions occurred during the nineteenth century. Thomas Hodgkin described the disease named after him in 1832. Hodgkin lymphoma is characterized by the presence in the lymphoid tissue of unique multinucleated Reed–Sternberg cells. Other examples of lymphoma lacking Reed–Sternberg cells were described and classified as non-Hodgkin lymphoma. Rudolf Virchow introduced the term leukemia in 1847 to describe a disease characterized by circulating malignant cells of the blood. Lymphocytic leukemia refers to those pathological conditions in which lymphocytes are transformed. In 1844 Samuel Solly described patients with a clinical syndrome he termed “mollities ossium” (softening of the bones) that we now designate multiple myeloma, one of the plasma cell dyscrasias. Finally, in 1944 Jan Waldenstrom described two patients who presented with a cancer of antibody-producing lymphocytes; this Waldenstrom macroglobulinemia is characterized by malignant transformation of IgM-synthesizing B lymphocytes. The study of these malignancies has provided a wider understanding of the maturation of the cells of the adaptive immune system and the genetic mechanisms involved in activating and regulating their response.

Chapter 37 – Tumor Immunology

At least three times during the history of immunology, investigators proposed that a function of the immune response is to eliminate nascent tumors. Paul Ehrlich initially hypothesized that tumors arise spontaneously in the body and that the immune response protected the individual against the majority of these malignancies. Little experimental data existed to support this hypothesis and were forgotten by most scientists working in this area. In the 1950s Lewis Thomas and F. Macfarlane Burnet individually postulated that lymphocytes recognize and respond to potential tumors; they termed this idea immunosurveillance. With the demonstration of separate populations of T and B lymphocytes, immunosurveillance was thought to be the responsibility of T lymphocytes. However, discrepancies in tumor incidence in T lymphocyte-deficient mice and humans compared to immunologically intact individuals cast doubt on this theory. The discovery of other mechanisms for the elimination of foreign pathogens, including natural killer lymphocytes, cytokine networks, and inflammation, stimulated a resurgence of interest in the concept that the immune system protects against tumor growth. This contemporary understanding of the immune system focused efforts to devise new strategies of immunotherapy for tumors.

Antibody-Mediated Effector Mechanisms

The production of antibody and the activation of specifically sensitized T lymphocytes comprise the two mechanisms used by the adaptive immune system to eliminate potentially pathogenic microorganisms. Antibodies bind specifically to structural conformations of the inducing antigen and activate components of the innate host defense system to protect the individual using one of the following effector mechanisms: • neutralization, a process in which antibody combines with antigen and inhibits it from binding to and invading susceptible cells of the body;

The Thymus in Lymphocyte Maturation

The immunological function of the thymus remained an enigma until 1961. Most biologists in the first half of the twentieth century considered the thymus to be either a vestigial organ or part of the endocrine system. Jacques Miller, investigating the mechanism by which murine leukemia virus induced leukemia, unexpectedly discovered that mice thymectomized at or shortly after birth failed to thrive and often succumbed to microbial infections. This observation prompted him to conclude that “the thymus at birth may be essential to life.” Subsequent studies showed that neonatally thymectomized mice have decreased lymphocyte counts in their peripheral blood and fail to reject foreign skin grafts. Some studies also demonstrated a defect in antibody production. Confirmation of an immunological role for the thymus came from clinical observations of individuals with a congenital absence of the thymus resulting in decreased immunocompetence and from studies on a strain of mice with congenital absence of a thymus. Along with the discovery of the role of the chicken bursa of Fabricius in the maturation of lymphocytes that synthesize and secrete antibodies, these studies led to the categorization of lymphocytes into functionally distinct T and B populations and the subsequent identification of several rare human immunodeficiency diseases.

Generation of Diversity in the Adaptive Immune Response

The adaptive immune system responds specifically to a large number of different antigenic determinants. F. Macfarlane Burnet proposed the clonal selection theory of antibody formation in 1959. One postulate of this theory is that antibody-forming lymphocytes (and by extension T lymphocytes) respond to a single antigen and that the specificity of these lymphocytes is determined during development prior to interaction with antigen. The mechanism by which lymphocytes generate this extensive repertoire of specificities remained an enigma until amino acid sequences and the molecular genetics of the formation of B and T lymphocyte receptors were determined. In the mid-1960s, William Dreyer and J. Claude Bennett hypothesized that immunoglobulin chains are coded for by two separate genes. Amino acid data confirmed that heavy and light chains had variable (V) and constant (C) regions. By 1976, Susumu Tonegawa and colleagues demonstrated that gene rearrangement involving separate genes coding for the V, diversity (D), joining (J), and C regions occurs during B lymphocyte development. Once the mechanism of gene rearrangement was understood for B lymphocytes, a similar mechanism in T lymphocytes was described. This process of gene rearrangement provides the lymphocyte with a unique set of specificities that are randomly determined and that allow the host to interact with virtually any antigen found in its environment.

B Lymphocyte Activation

The adaptive immune system produces antibodies to a large variety of foreign pathogens. The acceptance of the clonal selection theory of antibody formation and the division of lymphocytes into functional populations of T and B lymphocytes spurred studies on the mechanism of antibody formation by B lymphocytes. At a minimum, B lymphocytes must recognize the foreign antigen through a membrane-bound immunoglobulin, the B cell receptor. For a few T-independent antigens this interaction is sufficient to induce an IgM response. The response to most antigens, however, requires the interaction of B lymphocytes with antigen plus a second signal. A two-signal model of B lymphocyte activation was initially proposed by Peter Bretscher and Melvin Cohn in 1968. By the late 1960s, immunologists knew that optimal antibody formation required help from T lymphocytes. During the 1970s and 1980s evidence showed that this help came from two sources: direct interaction between T and B lymphocytes and through the intermediary of soluble cytokines. The T lymphocyte-derived signals activate synthesis and secretion of antibody and induce genetic changes, leading to the synthesis of five different isotypes (classes) of antibody possessing a variety of effector functions, all with the same antigenic specificity.

Defects in the Adaptive Immune Response Leading to Recurrent Infections

The adaptive immune system functions efficiently to eliminate potential pathogens that escape innate host defenses. Unfortunately, defects of the adaptive system can result in overwhelming infections with a variety of microorganisms. These defects may be either congenital or acquired. Prior to the availability of antibiotics, sporadic observations of children with histories involving recurrent infections appeared in the French, German, and English literature. The first description of a congenital immunodeficiency disease is generally attributed to Col. Ogden Bruton in 1952 with his publication of a case report of an 8-year-old boy with X-linked agammaglobulinemia. Since 1952 a large number of patients with different congenital immunodeficiencies have been reported: these include failure of the thymus to develop or function properly, antigen-presenting cells that failed to HLA molecules, and individuals presenting with a total lack of both T and B lymphocyte-mediated immunity. All of these diseases are rare. Finally in 1981 a report of recurrent and unusual infections in a group of homosexual males ushered in the realization that immunodeficiency could be acquired as a result of a viral infection. The information obtained from the study of both congenital and acquired immunodeficiencies has provided a wealth of information about the development and functioning of the normal immune system.