Correspondence

Regeneration of T Cells after Chemotherapy

N Engl J Med 1995; 332:1650-1652June 15, 1995

Article

To the Editor:

Mackall et al. (Jan. 19 issue)1 clearly demonstrate an inverse relation between age and the ability to regenerate CD4+ populations of T lymphocytes after intensive chemotherapy. They suggest that involution of the thymus (which occurs naturally with age) is likely to account for the diminished capacity to produce CD4+ cells. In the light of what is now known about extrathymic T-cell differentiation, however, the conclusion that “rapid T-cell regeneration requires residual thymic function” cannot be made without first examining CD8+ cell populations, which may have differentiated without a thymic influence. Studies in mice have shown that T-lymphocyte maturation can occur independently of the thymus in the small intenstine2 and liver.3 More recently, evidence of extrathymic T-cell differentiation has been found in the endometrium4 and small intestine5 in adults. These cells appear to be predominantly CD8+ and express homodimers of the CD8α chain, although subpopulations of CD4+CD8αβ+ intestinal T cells can originate in intestinal epithelium. The importance of this pathway of T-cell differentiation is unknown, but it may play a part as people age and thymic influences decrease. Extrathymic pathways of T-cell differentiation may therefore assume a role of central importance in adults who have undergone chemotherapy.

Although the small intestine can support positive and negative selection of T cells, not all subpopulations of T cells that have differentiated independently of the thymus undergo a deletion of autoreactive clones in the classic fashion of thymic T cells. It is therefore possible that a changing ratio of thymic T cells to extrathymic T cells accounts for the increased incidence of autoreactive clones in older people. Although there is a high prevalence of autoreactivity among the elderly, however, the actual incidence of autoimmune disease in this population is low. Increased levels of HLA-DR+ T cells in the circulation and changes in the CD4:CD8 and CD45RA:CD45RO (RA:RO) ratios are characteristic of the aging immune system. Thus, involution of the thymus is associated with alterations in the phenotype and function of circulating T cells, but contrary to current opinion, the well-being of the aging person is not necessarily immunologically compromised.6

The central role of the thymus in the production of T lymphocytes has been part of immunologic dogma for more than 30 years. A shift in our thinking about how and where T cells differentiate may change our thinking about autoimmune disease, the immunology of aging, and lymphocyte regeneration after chemotherapy and after transplantation.

Gaye Cunnane, M.B.
Cliona O'Farrelly, Ph.D.
St. Vincent's Hospital, Dublin 4, Ireland

6 References

1. 1

   Mackall CL, Fleisher TA, Brown MR, et al. Age, thymopoiesis, and CD4+ T-lymphocyte regeneration after intensive chemotherapy. N Engl J Med 1995;332:143-149
   Full Text | Web of Science | Medline

2. 2

   Rocha B, Vassalli P, Guy-Grand D. Thymic and extrathymic origins of gut intraepithelial lymphocyte populations in mice. J Exp Med 1994;180:681-686
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   Makino Y, Yamagata N, Sasho T, et al. Extrathymic development of Vα14-positive T cells. J Exp Med 1993;177:1399-1408
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   Hayakawa S, Saito S, Nemoto N, et al. Expression of recombinase-activating genes (RAG-1 and 2) in human decidual mononuclear cells. J Immunol 1994;153:4934-4939
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   Lynch S, Kelleher D, McManus R, O'Farrelly C. RAG-1 and RAG-2 expression in human intestinal epithelium: evidence of extrathymic T cell differentiation. Eur J Immunol 1995;25:1143-1147
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   Franceschi C, Monti D, Sansoni P, Cossarizza A. The immunology of exceptional individuals: the lesson of centenarians. Immunol Today 1995;16:12-16
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To the Editor:

Mackall et al. report that the rate of recovery of CD4+ cells after chemotherapy is dependent to some extent on the thymus. This conclusion is based on the assumption that the thymus is the source of CD45RA+ cells (which express the longest isoform of CD45). Other observations cast their data in a different light.

First, rat lymphocytes physiologically revert from the expression of shorter isoforms to the expression of the longest isoform of CD45.1 In vivo measurements of the life span of human lymphocytes, with the use of the CD45RO and CD45RA isoforms, support the concept of a process of reversion in humans.2,3 The CD45RA+ cells identified in the study by Mackall et al. may therefore be produced by the reversion of CD45RO+ cells, as well as by any thymic tissue present. Reversion, which may take place at different rates in patients of different ages, has been used to account for the appearance of CD45RA+ cells after bone marrow transplantation.4

Second, it has been shown that CD45RO+ cells divide more often than do CD45RA+ cells.2,3 With this knowledge and the recognition that a reversion from the RO phenotype to the RA phenotype is possible, the complexity of the relation between the RA:RO ratio and the growth rate becomes apparent. On the one hand, the immigration of CD45RA+ cells from the thymus suggests a higher growth rate and thus a higher RA:RO ratio. On the other hand, higher proliferation rates among CD45RO+ cells lead to a higher growth rate and thus a lower RA:RO ratio. The proliferation of CD45RO+ cells and subsequent reversion therefore represent a potential confounding factor in the relation between the RA:RO ratio and the rate of growth of CD4+ lymphocytes, which may explain why Mackall et al. found a rather low correlation coefficient (r = 0.64) between the two.

Figure 1Figure 1Relation between the RA:RO Ratio and the Change in the T-Cell Count after Six Months of Growth. shows the predicted relation between the RA:RO ratio and the change in the T-cell count at the end of six months of growth, calculated with the use of a mathematical model3 of the growth of CD45RA and CD45RO cells. The relation is shown for four fixed rates of immigration from the thymus. We assumed that our hypothetical patient had a T-cell count of 95 cells per cubic millimeter and that those cells were initially in an RA:RO ratio that ranged from 1:94 to 94:1. We then used our model and its estimated values to predict the change in the number of T cells and the RA:RO ratio after six months of growth. The figure shows that for a fixed rate of immigration, the rate of T-cell regrowth rises as the RA:RO ratio falls.

The conclusion reached by the authors, that “rapid T-cell regeneration requires residual thymic function,” might therefore be rephrased to emphasize that factors other than the presence of the thymus contribute to the rate of recovery of the CD4+ subgroup. Further analysis of these other factors is required for an informed debate about their potential clinical relevance.

Colin A. Michie, M.R.C.P.
Ealing Hospital NHS Trust, Ealing, Middlesex UB1 3HW, United Kingdom

Angela R. McLean, Ph.D.
Institut Pasteur, 75724 Paris, France

4 References

1. 1

   Sparshott SM, Bell EB, Sarawar SR. CD45R CD4 T cell subset-reconstituted nude rats: subset-dependent survival of recipients and bi-directional isoform switching. Eur J Immunol 1991;21:993-1000
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2. 2

   Michie CA, McLean A, Alcock C, Beverley PC. Lifespan of human lymphocyte subsets defined by CD45 isoforms. Nature 1992;360:264-265
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3. 3

   McLean AR, Michie CA. In vivo estimates of division and death rates of human T lymphocytes. Proc Natl Acad Sci U S A 1995;92:3707-3711
   CrossRef | Web of Science | Medline

4. 4

   Michie CA, McLean AR. Bone marrow transplantation and lymphocyte subset kinetics. Bone Marrow Transplant 1993;12:547-547
   Web of Science | Medline

To the Editor:

We have examined the reconstitution of CD4+ T lymphocytes in patients with rheumatoid arthritis treated with a depleting monoclonal antibody to CD4+ T cells (cM-T412).1,2 Severe, prolonged CD4+ T-cell depletion was observed in patients treated with cM-T412.2 Prolonged depletion of CD4+ T cells in rheumatoid arthritis has aroused concern about the ability of the immune system to reconstitute CD4+ T lymphocytes in patients with rheumatoid arthritis.

We have conducted extended studies to investigate prolonged CD4+ T-cell depletion in patients with rheumatoid arthritis.3 Although the depletion of CD4+ T cells was sustained in the majority of our 23 patients 18 and 30 months after the infusion of cM-T412, a partial repopulation of CD4+T cells (>50 percent increase between 14 days and 6 months after infusion) was observed in approximately 40 percent of the patients, involving combined increases in both CD45RO+(memory) and CD45RA+(naive) subpopulations of CD4+ T cells (Table 1Table 1CD41 T-Cell Counts after Treatment with a Monoclonal Antibody (cM-T412) in 23 Patients with Rheumatoid Arthritis.).3 The deficiency of CD4+ T cells was not accompanied by a compensatory increase in CD8+ T cells, which remained essentially unchanged from preinfusion values. Thus, the total CD3+ T-cell count was also decreased. Neither the patient's age nor the dose of cM-T412 was correlated with the degree of repopulation, although no children were included in our study.

The incomplete reconstitution of CD4+ T cells in these patients suggests that care must be exercised in selecting the type of T-cell–directed monoclonal antibody (depleting or nondepleting) in the treatment of rheumatoid arthritis and other autoimmune disorders. The choice of depleting monoclonal antibody such as cM-T412 may preclude the use of an otherwise appropriate therapeutic dose. Data from animal models of autoimmune disease indicate that the efficacy of an anti-CD4 monoclonal antibody is not dependent on the physical depletion of T cells, suggesting that the modulation of T-cell function (e.g., tolerance induction) is more likely to be responsible for the effectiveness of anti-CD4 monoclonal antibodies.4 Indeed, autoimmunity in NZB–NZW mice can also be suppressed by the administration of nondepleting F(ab')₂ fragments of anti-CD4 monoclonal antibody.4 Studies of nondepleting anti-CD4 monoclonal antibodies in rheumatoid arthritis, which are in progress, should be helpful in resolving these important issues.5

Our data support the conclusion that the capacity to generate new T cells after their depletion by therapeutic agents is very limited in adults. This limitation may have important implications for other conditions associated with T-cell depletion, such as infection with the human immunodeficiency virus.

Larry W. Moreland, M.D.
R. Pat Bucy, M.D., Ph.D.
William J. Koopman, M.D.
University of Alabama at Birmingham, Birmingham, AL 35294

5 References

1. 1

   Moreland LW, Bucy RP, Tilden A, et al. Use of a chimeric monoclonal anti-CD4 antibody in patients with refractory rheumatoid arthritis. Arthritis Rheum 1993;36:307-318
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2. 2

   Moreland LW, Pratt PW, Bucy RP, Jackson BS, Feldman JW, Koopman WJ. Treatment of refractory rheumatoid arthritis with a chimeric anti-CD4 monoclonal antibody: long-term followup of CD4+ T cell counts. Arthritis Rheum 1994;37:834-8
   

3. 3

   Pratt PW, Bucy RP, Moreland LW, Koopman WJ. Thymic processing appears to contribute to the repopulation of CD4 lymphocytes after depletion with chimeric CD4 antibody in refractory rheumatoid arthritis. Arthritis Rheum 1992;35:Suppl:S105-S105 abstract.
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4. 4

   Carteron NL, Wofsy D, Seaman WE. Induction of immune tolerance during administration of monoclonal antibody to L3T4 does not depend on depletion of L3T4+ cells. J Immunol 1988;140:713-716
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5. 5

   Moreland LW, Bucy RP, Knowles RW, et al. Treating rheumatoid arthritis with a non-depleting anti-CD4 monoclonal antibody (MAb). J Investig Med 1995;43:Suppl:362A-362A abstract.
   

Author/Editor Response

The authors reply:

To the Editor: The data provided by Dr. Moreland and his colleagues, showing impaired CD4+ T-lymphocyte regeneration in adults receiving treatment with anti-CD4 monoclonal antibodies, are consistent with our observations and indicate that our results are not unique to chemotherapy-induced CD4+ lymphocytopenia.

Drs. Cunnane and O'Farrelly report evidence of extrathymic maturation of CD8+ T-cell populations. We too have observed thymic-independent generation of CD8+ cells from bone marrow progenitors in a murine model.1 Among the patients described in our recent report, the correlation between age and the CD8+ count six months after chemotherapy was weaker than the correlation between age and the CD4+ count. We hypothesize that although thymic maturation has a role in CD8+ T-cell regeneration, extrathymic pathways play a part as well in humans. In contrast, there is little evidence that extrathymic maturation makes a sizable contribution to CD4+ T-cell regeneration (the focus of our report).

The data noted by Drs. Michie and McLean describing CD45 isoform switching in CD4+ T-cell populations in rats2 raises the important issue of the quantitative contribution of the CD4+CD45RO+ reverted cells to the total CD4+CD45RA+ pool during T-cell regeneration. The conclusions in our report were based on relations among age, CD4+CD45RA+ cells, and thymic size in humans as well as on previous experiments with animals. In our murine model, the reversion of CD4+ cells of the memory cell-surface phenotype to a naive cell-surface phenotype is extremely limited.1 The hypothesis that the reversion of CD4+CD45RO+ cells to CD4+CD45RA+ cells was primarily responsible for our observations in humans would require that reversion occur to a greater extent in younger patients and be correlated with thymic size. Furthermore, because the appearance of CD45RA+ cells was correlated with an increase in CD4+ T cells, the reversion would have to occur at a substantial rate in dividing populations. This is inconsistent with extensive data indicating CD45RO expression in dividing T-cell populations.3 Thus, although it is impossible to eliminate the possibility that isolated CD4+CD45RA+ cells exist that were derived from CD4+CD45RO+ cells, we believe the most plausible explanation for the data as a whole is that CD4+CD45RA+ cells exported from the thymus largely account for the regeneration of CD4+ T cells. The relevance of the mathematical model to our observations is unclear and would require further testing for validation.

Crystal L. Mackall, M.D.
Seth M. Steinberg, Ph.D.
Ronald E. Gress, M.D.
National Cancer Institute, Bethesda, MD 20892

3 References

1. 1

   Mackall CL, Granger L, Sheard MA, Cepeda R, Gress RE. T-cell regeneration after bone marrow transplantation: differential CD45 isoform expression on thymic-derived versus thymic-independent progeny. Blood 1993;82:2585-2594
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2. 2

   Bell EB, Sparshott SM. Interconversion of CD45R subsets of CD4 T cells in vivo. Nature 1990;348:163-166
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3. 3

   Cerottini JC, MacDonald HR. The cellular basis of T-cell memory. Annu Rev Immunol 1989;7:77-89
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Citing Articles (2)

Citing Articles

1. 1

   CLIVE M. GRAY, JONATHAN M. SCHAPIRO, MARK A. WINTERS, THOMAS C. MERIGAN. (1998) Changes in CD4 ⁺ and CD8 ⁺ T Cell Subsets in Response to Highly Active Antiretroviral Therapy in HIV Type 1-Infected Patients with Prior Protease Inhibitor Experience. AIDS Research and Human Retroviruses 14:7, 561-569
   CrossRef

2. 2

   Russell W. Anderson, Michael S. Ascher, Haynes W. Sheppard. (1998) Direct HIV Cytopathicity Cannot Account for CD4 Decline in AIDS in the Presence of Homeostasis: A Worst-Case Dynamic Analysis. Journal of Acquired Immune Deficiency Syndromes and Human Retrovirology 17:3, 245-252
   CrossRef