Case 32

Published on 18/02/2015 by admin

Filed under Allergy and Immunology

Last modified 22/04/2025

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CASE 32

You are looking after a patient with B cell chronic lymphocytic leukemia (CLL). There are a number of conventional chemotherapeutic regimens available to treat this disease and a number of other investigational ones. Included among the investigational therapies is one involving infusion of antibodies made artificially (ex vivo) to the patient’s own tumor cells. There is, in fact, a major accepted immunologic dogma behind this treatment and why it might be expected to work, although thus far in practice such therapy has not been as effective as we might have hoped.

QUESTIONS FOR GROUP DISCUSSION

1. In B cell CLL, membrane immunoglobulin (B cell antigen receptor) is considered a tumor-specific antigen. Explain.
2. There are two major types of CLL. Explain how these populations differ.
3. In what types of cells is ZAP-70 normally found? In general terms, what is its role?
4. How does CLL typically present clinically? How is it diagnosed?
5. Assuming that an antibody can be generated that is specific for the tumor-specific antigen (question 1), the immune complex that is formed on the cell surface could activate a number of immunologic processes to destroy these malignant cells. Explain how each of the following could be recruited to destroy the malignant B cells: (a) complement, (b) neutrophils, and (c) NK cells.
6. One would expect that the infusion of tumor-specific antibodies would lead to the effective killing and elimination of malignant cells. However, this approach has not been particularly effective. Explain why that might be the case (see Tumor Immunology, Section IV).
7. Describe the “Jerne idiotypic network theory.” How would these networks produce the normal homeostasis in both T and B lymphocytes within the immune system?
8. Assuming that you could develop the appropriate anti-idiotypic antibody to target a particular population of T cells, what in vitro experiments could you perform that would provide information as to the potential efficacy of this approach?
9. The generation of antibodies specific for the idiotype on B cells would, in effect, represent the “magic bullet” therapy so long sought. The effectiveness of this therapy relies on the individual uniqueness (magic bullet) of the treatment. Yet, this is also its major limitation as a therapeutic approach in clinical practice. Explain.
10. Why would the use of T cell or B cell differentiation molecules as targets for antibodies tagged with toxins/radioactive material be more attractive for therapy than similarly labeled anti-idiotypic antibodies in clinical practice?

RECOMMENDED APPROACH

Implications/Analysis of Family History

B cell CLL is a monoclonal tumor, resulting from the proliferation and expansion of a single clone of B cells at a “frozen” point in time in their development. Studies of CLL and family history have shown an elevated risk of CLL for offspring and siblings of patients with CLL, as well as first- and second-degree relatives. However, no specific gene has been identified that is the cause of the disease per se. Recent literature reports have focused on the fact that patients with CLL can be divided into two major groups: those that have mutations in the variable region of the immunoglobulin heavy chain (VH) and those that do not have mutations in this gene. Patients with the mutated gene have a better prognosis than those without the mutation. It is now known that there are about 244 genes that are differently expressed in these two types of CLL. Of these 244 genes, ZAP-70 has been determined to be the best prognostic marker. ZAP-70, a tyrosine kinase, is a key signaling molecule in T cells and NK cells. It is not normally expressed in B cells. However, it is expressed in most patients with unmutated immunoglobulin VH gene, where it is recruited to the B cell receptor and enhances signaling.

Implications/Analysis of Clinical History

More than half the patients diagnosed with CLL are asymptomatic. For these patients diagnosis is made after a blood test for other reasons, including a yearly physical examination. Patients who are symptomatic will present with fatigue, weight loss, and generalized lymphadenopathy. The median age of CLL diagnosis is 64 years.

Implications/Analysis of Laboratory Investigation

An increased white blood cell count with more than 90% small lymphocytes (normal is about 30%) is suggestive of CLL. Bone marrow biopsy will generally show evidence for altered differentiation (myelodysplasia).

Additional Laboratory Tests

Immunotyping of cells using flow cytometry is also important in diagnosis. Panmarkers CD19 and CD20 will be present, as will CD5, CD43, and CD23. In this case, these cells were shown to be ZAP-70 negative, as determined by flow cytometry.

DIAGNOSIS

A diagnosis of B cell chronic lymphocytic leukemia was made. This patient was determined to have a mutated VH immunoglobulin gene and was negative for ZAP-70, suggesting a good prognosis.

STAGING AND PROGNOSIS

Two systems have been used classically to determine clinical staging and prognosis. The Rai system is based on the idea that nonfunctioning lymphocytes accumulate with time. There are four stages, with the first stage indicating increased numbers of lymphocytes in the bone marrow and in circulation. Stages II, III, and IV include enlarged spleen, anemia, and thrombocytopenia, respectively, along with the lymphocytosis referred to in stage I. The Binet system is similar but includes the number of enlarged lymphoid areas. Recent studies indicate that a better approach to predict progression of CLL is the detection of ZAP-70 in B cells, using flow cytometry for assessment.

THERAPY

There are a number of conventional chemotherapeutic regimens available to treat this disease and a number of other investigational ones (see later).

ETIOLOGY: CHRONIC LYMPHOCYTIC LEUKEMIA

CLL is a monoclonal tumor resulting from the proliferation and expansion of a single clone of B cells at a “frozen” point in time in their development. All tumor cells of the malignant clone are identical and express the same immunoglobulin molecule on their surface, and this immunoglobulin is unique to this B cell clone alone (an accepted principle of generation of diversity in the immune system). Thus the surface immunoglobulin on this cloned population is an example of a tumor-specific antigen.

Surface immunoglobulins are antigens expressed uniquely on a subpopulation of B cells. Therefore, if reagents (antibodies) could be made to it they would, in principle, be ideal targets in that only the tumor cells would be recognized.

Investigational Chemotherapeutic Regimens

Included among the investigational therapies is one involving infusion of antibodies made artificially (ex vivo) to the patient’s own tumor cells. There is, in fact, a major accepted immunologic dogma behind this treatment, and why it might be expected to work, although thus far in practice such therapy has not been as effective as we would have hoped.

Mechanism of Tumor Destruction Using Monoclonal Antibody Therapy

As a result of such recognition, several, again conventional, immunologic reactions should occur:

1. Complement mediated lysis would be expected to destroy tumor cells (after activation of MAC, and osmotic lysis of cells). This has indeed been shown for some leukemic cells in vitro.
2. Increased phagocytosis of tumor cells by host cells with Fc (or C3) receptors should result in destruction by cells of the innate immune system.
3. Antibody-dependent cell-mediated cytotoxicity, after recognition by Fc or C3 receptor-bearing cells, even in the absence of phagocytosis, should suffice to activate either killing machinery, or growth-inhibitory machinery in the host cells, a mechanism referred to as ADCC. NK cells are believed to be particularly effective in this phenomenon.

Limitations to Antibody Therapy

Antigenic Modulation

As noted earlier, in general these mechanisms of tumor protection have not proven as effective in practice as in theory. One explanation for this has been that antibody-mediated downregulation of surface antigen (antigen modulation) occurs to render the tumor refractory to cell killing. The net observed result (outgrowth of populations of tumor cells with lower/absent surface immunoglobulin) could reflect a passive selection process (antibody merely allows for the selective “darwinian” outgrowth of [preexisting mutant] cells with little surface immunoglobulin), or an active process, resulting from engagement of surface immunoglobulin by the anti-immunoglobulin antibody and subsequent internalization of the complex with reduced re-expression in so-called tumor cell revertants. Whichever the mechanism, the loss of surface immunoglobulin makes these cells refractory to such treatment. It also unfortunately makes it less easy to monitor the presence of disease (decreased numbers of the surface immunoglobulin–marked tumor clone).

Individual Uniqueness

A major problem with consideration of this form of therapy in clinical practice is the very individual uniqueness of the treatment. What will work (be effective for) CLL with idiotype 1 will not work for CLL with idiotype 2. The cost and time frame of development of such tailored (to the individual) therapy is prohibitive.

Developmental Stage-Specific Differentiation Markers

This, in turn, has focused attention on the potential role of developing reagents directed against more common (e.g., developmental-specific) antigens. When B cells (and indeed) T cells, differentiate, they express unique stage-specific differentiation markers. There is some hope that reagents directed against these may be used alone, or after suitable tagging (with toxins/radioactive materials, etc.), to serve as diagnostic/immunotherapeutic tools.

Novel Genes

Finally, we have slowly come to realize that alterations in the normal patterns of differentiation that result in malignant transformation is itself often associated with overexpression of novel genes that can themselves serve as suitable targets for immunotherapeutic intervention. Kinase genes (e.g., ZAP-70), and other oncogenes, have proven to be just such molecules, and there is a growing interest in using both as target for either conventional pharmacotherapy and/or immunotherapy.

Jerne’s Idiotypic Network Theory

As mentioned, the concept of producing antibody (or immunoreactive lymphocytes) to antigen-specific cell surface receptors on other immune cells forms the basis of a concept/theory of normal immunoregulation, the so-called Jerne’s idiotypic network theory.

Jerne’s idiotypic network theory suggests that normal immune regulation involves this mutual recognition of surface immunoglobulin, or T cell receptor, determinants by a network of interacting lymphocytes, producing the normal homeostasis in both B cells and T cells in the immune system. Jerne postulated that only by envisaging such a network could one imagine a realistic closed regulated immune system. Without it the system would be open ended. For each and every cell with a unique (idiotypic) surface immunoglobulin (or T cell receptor) Jerne postulated the existence of a (regulatory) cell with a counter-receptor (an anti-idiotypic receptor).

Anti-Idiotypic Antibodies to Target B Cells or T Cells

For the patient discussed, what we are essentially trying to do is develop this anti-idiotypic population, which would thus regulate expansion/development of the tumor clone expressing an anti-idiotypic surface immunoglobulin. Because Jerne’s theory predicts idiotypy in T cells as well as in B cells, an alternative strategy might be to generate B cells whose surface immunoglobulin is specific for the idiotype of T cells that provide help for the tumorigenic B cell clone. An alternative approach is to produce B cells that are specific for the idiotype on the CLL cells.

Evidence for T Cell Immunity to CLL

Evidence for such immunization of T cells for anti-idiotypic regulation could be sought by exploring evidence for direct T cell immunity to CLL. Proliferation of T cells or production of cytokines CD4 cells after stimulation with the surface immunoglobulin (antigen) specific for the T cell idiotype could provide such evidence. Evidence for antigen-restricted recognition by T cells in vivo implies the expansion of a (antigen-limited) T cell pool. In general, evidence for antigen immunization in a T cell pool is sought by asking whether there is evidence for limited Vβ T cell expansion after the immunization procedure.

Animal Studies

At least in animal studies there is some evidence that anti-idiotypic immunization of T cell immunity may be (more) effective (than induction of anti-idiotypic antibody). Why this is so is not clear, although this may reflect the engagement of unique T cell functions in a (presumed) regulatory T cell population. Note that there is (in Jerne’s theory), however, no a priori difference (in initial function) between idiotype-bearing cells and anti-idiotype–bearing cells. Whichever is the “mirror” image depends on one’s viewpoint!

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