Mechanisms of T-Cell Depletion and Regeneration in Hivdisease Joseph M

Mechanisms of T-Cell Depletion and Regeneration in HIVDisease Joseph M. McCune, md, phd Senior Investigator, Gladstone Institute of Virology and Immunology Professor of Medicine and of Microbiology and Immunology, University of California, San Francisco San Francisco, California Reprinted from The PRN Notebook,™ september 2002. Dr. James F. Braun, Editor-in-Chief. Tim Horn, Executive Editor. Summary by Tim Horn Published in New York City by the Physicians’ Research Network, Inc.,® Edited by Marc Hellerstein, md, phd John Graham Brown, Executive Director. For further information and other articles available online, visit http://www.PRN.org All rights reserved. © september 2002.

ore than 20 years since the be- Ashley Haase, Director of the Microbiology turnover model) or because their produc- ginning of the aids epidemic, Department at the University of Minneso- tion is impaired (the regenerative failure there is still no clear-cut expla- ta, estimated that healthy young adults model) (see Figure 1). Another theory that nation for hiv’s most basic and harbor approximately 200 billion mature is being examined, although it was not insidious effect in the human CD4+ cells (Haase, 1999). In hiv-positive discussed by Dr. McCune, holds that the body: the gradual depletion of patients, Dr. Haase and others reckon that fraction of circulating cells may decrease— MCD4+ T-lymphocytes in the absence of an- this total number is halved by the time the giving the appearance of loss—if hiv in- tiretroviral treatment. At the same time, CD4+ cell count falls to 200 cells/mm.3 And fection results in their redistribution out of there is little consensus regarding the in more advanced disease, Dr. McCune the peripheral blood and into the confines mechanism(s) by which CD4+ cell counts pointed out that destruction of parenchy- of lymphoid organs. [Editor’s note: These improve once therapy is commenced. mal lymphoid spaces is so extensive that and other theories are discussed in a com- On some level, the fact that CD4+ cell enumeration of the total body CD4+ cell prehensive review article authored by Dr. counts do improve—both quantitatively count has not even been attempted. McCune, published in the April 19, 2001, and qualitatively—with antiretroviral ther- There are actually several plausible issue of Nature (McCune, 2001)]. apy should be enough to satisfy the hearts theories to explain the CD4+ cell depletion As discussed in several past issues of and minds of hiv-treating clinicians. But seen in hiv disease. As was discussed by The PRN Notebook, the high-turnover mod- there is much to be gained medicinally Dr. McCune, CD4+ cells may be depleted be- el—developed and maintained by groups with the continued exploration of these cause they are destroyed (the high- led by Dr. David Ho and others—concludes fundamental questions. For example, antiretroviral therapy plays a critical role in table 1. Mechanisms of cd4+ Cell Depletion: halting the accelerated destruction of CD4+ cells, perhaps the most widely accepted Destruction of Mature cd4+ Cells mechanism of hiv-associated CD4+ cell depletion. But there is also at least one oth- Direct destruction of infected cells er mechanism to ponder—the impaired • Envelope-mediated apoptosis production of new CD4+ cells as a result of • Vpr-induced g2 arrest and apoptosis hiv infection—that may have significant bearing on the evaluation and potential • Disruption of cell membrane integrity/syncytia formation use of various immune-based therapies • Accumulation of unintegrated viral dna in the clinical management of hiv-positive patients. Indirect induction of death in uninfected cells • Cytolysis by hiv-specific cytolytic T-cells or by natural killer cells • Autoimmune reactions of a humoral or cellular nature Understanding cd4+ Cell Depletion • Incorporation into syncytia by neighboring infected cells if there is one thing virtually all ex- • Triggering of apoptosis upon cell activation or cross-linking of cd4-bound gp120 perts agree on, it is that untreated hiv in- hiv fection results in the progressive loss of • Enhanced transmission and/or apoptosis following interaction with nearby CD4+ cells from the circulation as well as infected antigen-presenting cell depletion of CD4+ cells from total body Source: McCune, 2001. Reprinted with permission of Nature and Macmillan Magazines, Ltd. stores. In a paper published in 1999, Dr.

4 THE PRN NOTEBOOK™ • VOLUME 7, NUMBER 3 • SEPTEMBER 2002 • WWW.PRN.ORG that hiv induces CD4+ cell death in the pe- ﬁgure 1. riphery. A variation on this theme—pro- Accelerated Destruction and Regenerative Failure posed by Drs. Zvi Grossman of Tel-Aviv Models of hiv University, Frank Miedema of the Univer- sity of Amsterdam, and others—is that hiv Regenerative Failure Accelerated Destruction also induces a state of chronic immune (Low Turnover Model) (High Turnover Model) activation resulting in increased apoptosis Increase and CD4+ cell death. Upon initiation of A cd4 Bone Marrow haart, viral burden is dramatically re- Progenitors Decrease duced. Treatment, in turn, reduces immune activation, thus halting cell destruction. Some of the direct and indirect mech- High Viral anisms by which CD4+ cells are destroyed Load as a result of hiv infection—and halted as a result of haart—are listed in Table 1. Increased Death cd4 Memory Cells Become of cd4 Productively Infected A critical distinction between the mod- Progenitors els for high turnover and regenerative failure is that the latter includes a primary pathogenic event in hiv disease that rests Uncontrolled Viral Replication Viral Uncontrolled upon a failure to produce new CD4+ (and CD8 +) cells (Hellerstein, 1997). The model Naive cd4 Cells speciﬁes that the sources of production— cd4 Intrathymic Increased Death namely, the bone marrow, the thymus, Progenitors of Mature cd4 Cells and other extrathymic lymphoid organs B cd4 Bone Marrow (e.g., lymph nodes, spleen, and mucosa)— Progenitors HAART are rendered dysfunctional in later stages of hiv disease, resulting in low (or no) rates of replacement in the face of continued CD4+ cell destruction. Over time, this Low imbalance leads to a decrease in the total Viral Load body pool of mature T-cells and to im- Decreased Death Decreased Turnover of mune system collapse, whether or not the haart of cd4 Infected cd4 Memory Cells Progenitors destruction rate of CD4+ cells is accelerat-

ed. With the initiation of haart, the viral Effect of assault on progenitor cells is halted, al- lowing for the production of new T-cells. Coupled with diminished destruction of Increased Production of Naive mature T-cells, the CD4+ cell count rises. and Memory cd4 Cells cd4 Decreased Death of In quantitative terms, the crux of the Intrathymic cd4 Progenitors Mature Cells debate between these two models is framed by two parameters: 1) the “frac- When hiv is introduced into the body (Figure 1A), it is probably concentrated with drain- tional replacement rate”—or the fraction of new T-cells made and lost each day—a ing lymph nodes and presented as an antigen, resulting in enhanced movement of T-cells term that is inversely related to the cell into nodes and vigorous T-cell proliferation. The high turnover model holds that immune half-life (at steady state, the higher the activation caused by hiv will be associated, by direct and indirect means, with acceler- fractional replacement rate of a given cell ated destruction of T-cells and that the initiation of highly active antiretroviral therapy subpopulation, the shorter its half-life); (Figure 1B) slows T-cell turnover and with it a decrease in the death rate of cd4+ cells and 2) the “absolute production rate”— (right halves of Figure 1). The regenerative failure model (left halves of Figure 1) spec- or the estimated total number of new cells iﬁes that the source of T-cell production—namely the bone marrow, the thymus, and oth- made each day—of a given cell subpopu- er extrathymic lymphoid organs—are rendered dysfunctional in later stages of hiv dis- lation. The former parameter tells how ease, resulting in low (or no) rates of replacement in the face of continued cd4+ cell de- long each cell in the body will live, on av- struction. Over time, this imbalance leads to a decrease in the total pool of mature T-cells erage; the latter tells how many new cells and to immune system collapse, whether or not the destruction rate of cd4+ cells is ac- the body makes each day. celerated. With the initiation of haart, the viral assault on progenitor cells is halted, al- The difference between these two parameters may be easier to understand by lowing for the production of new T-cells. Coupled with diminished destruction of mature cd4 drawing upon a more simple analogy: in- T-cells, the + cell count rises. terest on a car loan. A fractional interest Source: McCune, 1999. Reprinted with permission of HIV: Advances in Research and Therapy and Cliggott Publishing Company. rate (e.g., 10% per year on a car loan) im- Based on an illustration by Neil Harris for Cliggott Publishing Company.

THE PRN NOTEBOOK™ • VOLUME 7, NUMBER 3 • SEPTEMBER 2002 • WWW.PRN.ORG 5 plies the length of time it will take to pay the deuterated substrate (glucose or wa- creased. After viral replication was sup- double the requested loan amount (i.e., ter)—permitting a direct estimate of the pressed by haart in Group 3 subjects for 12 ten years). The absolute interest rate (e.g., fraction of cells that divided during the weeks, the fractional replacement rate of T- $2,000 on a $20,000 car) involves the ac- period of label administration. A high frac- cells was not significantly different than tual amount of money a household must tion indicates that there has been a sig- those recorded in Group 2. Importantly, generate to stay in balance with the loan nificant amount of cellular proliferation; if the kinetic basis for increased CD4+ T-cell payments. Both of these parameters might the fraction is low, proliferation has been levels was greater absolute production, not be considered when someone decides limited. Given information about the num- a longer half-life, of circulating cells. whether to take out a loan. ber of circulating T-cells, it is possible to These parameters apply to hiv infection calculate the absolute production rate of and CD4+ cell counts. The high turnover newly made T-cells in the circulation. model, in its simplest form, holds that In the 1999 study reviewed by Dr. Mc- Mechanisms of high levels of virus drive a high fractional Cune, three groups of subjects were evalu- Regenerative Failure replacement rate, by increasing the likeli- ated: Group 1 consisted of nine hiv-negative it is important to remember that pro- hood that a given CD4+ cell will die. Low study volunteers with a mean CD4+ cell gressive hiv disease is not simply charac- levels of virus result in a lower likelihood count of 1300 cells/mm3; Group 2 included terized by the development of lymphope- of cell death, and a lower fractional re- seven untreated or ineffectively treated hiv- nia. Patients with aids often face a myriad placement rate. The regenerative failure positive individuals with a mean CD4+ T-cell of other complications, including anemia, model maintains that CD4+ cell counts drop count of 342 cells/mm3; and Group 3 was neutropenia, and thrombocytopenia. These because the absolute rate of cell production made up of five individuals initiating haart observations—which are certainly not new does not increase to compensate for in- with a mean CD4+ T-cell count of 184 to researchers or clinicians—suggest that creased CD4+ cell death. High levels of cells/mm.3 In the uninfected subjects of hiv infection profoundly affects multilin- virus reduce the number of cells produced Group 1, the CD4+ and CD8+ cell popula- eage and lineage-specific hematopoiesis, (the absolute production rate) more than tion had average half-lives of 87 days and which ultimately results in regenerative low levels of virus do. 77 days, respectively, with absolute pro- failure along several key cell lines (Table 2). Only recently has it become possible duction rates of 10 new, circulating CD4+ There is no shortage of evidence sug- to directly measure absolute production cells/mm3 per day and 6 new, circulating gesting that bone-marrow and intrathymic rates. While radioactive labeling has long CD8+ cells/mm3 per day. For those infected T-progenitor cells can be affected and de- been used in animal models, it has gen- but untreated subjects in Group 2, the half- pleted by hiv, at least in vitro. To explore erally been frowned upon for use in hu- life of each subpopulation was shorter than these observations in vivo, Drs. Morgan man beings. In 1998, a novel and much that observed in Group 1, but this increase Jenkins, McCune, and their colleagues at safer technique—pioneered by Dr. Marc in cell death was not offset by a compen- the Gladstone Institute investigated the Hellerstein and his colleagues at the Uni- satory increase in CD4+ cell production. In effects of hiv infection on early hematopoi- versity of California in both Berkeley and other words, the fractional replacement etic progenitor function in immune-defi- San Francisco—permits, for the first time, rate of CD4+ and CD8+ cells was higher cient mice with surgically implanted hu- the direct determination of fractional re- among individuals in Group 2 than that man thymus and liver tissues (the scid-hu placement rates of T-cells in humans. In seen in hiv-negative controls in Group 1, Thy/Liv model) (Jenkins, 1998). The group this technique, the dna of dividing cells but the absolute production rate was not in- reported that both lineage-restricted and is labeled with deuterated glucose (D-glu) or deuterated water (D20), compounds that table 2. Mechanisms of cd4+ Cell Depletion: are safe to administer to humans and are Impaired T-Cell Production incorporated directly in the deoxyribonu- cleosides of dna. Direct effects of virus Drs. Hellerstein, McCune and their colleagues at the Gladstone Institute have • Infection-mediated death of progenitor cells used this technique to measure the frac- • Destruction of the supporting stromal network required for multilineage or tional replacement rates of T-cells in hiv- lineage-restricted hematopoiesis uninfected control subjects and in untreated and treated hiv-infected subjects Indirect effects of virus (Hellerstein, 1999). After a labeling period, • Cytokine dysfunction circulating peripheral blood mononuclear • Opportunistic infections of bone marrow (e.g., cmv or mac) cells are drawn and separated by a cell hiv sorter into subpopulations, such as CD4+ • -induced apoptosis and CD8+ cells. Genomic dna from these • Infiltrating malignancies cells is then isolated and broken into its • Myelotoxic effects of drugs component nucleosides. Using mass spec- trometry, it then becomes possible to mea- • Deficiences of vitamins and of other essential factors sure the fraction of newly made dna— Source: McCune, 2001. Reprinted with permission of Nature and Macmillan Magazines, Ltd. which contains a chemical signature from

6 THE PRN NOTEBOOK™ • VOLUME 7, NUMBER 3 • SEPTEMBER 2002 • WWW.PRN.ORG multilineage hematopoietic figure 2. to the opposite conclusion: The progenitors were depleted Thymus is Abundant in Some thymus appears to be very much from the human Thy/Liv grafts hiv-Positive Adults present and functional in a sur- infected with either a molecu- prisingly large percentage of hiv- lar clone or a primary isolate P = 0.03 hiv-Negative infected patients, including those of hiv. Depletion of hematopoi- > 500 40 years of age and older (Mc- 300–500 hiv-Positive etic progenitors occurred sev- cd4+ Cune, 1998). 80 100–300 Cell Count eral days before the onset of < 100 As described in a 1998 re- thymocyte depletion, indicat- P = 0.03 port published in the Journal of 60 ing that the subsequent rapid Clinical Investigation, Dr. Mc- decline in thymocyte numbers 40 Cune’s Gladstone Institute group was due at least in part to loss recruited 99 hiv-positive patients of thymocyte progenitors. hiv- 20 and 32 hiv-negative controls to dna was not detected at high undergo ct scans, which were frequency in hematopoietic Thymus with Abundant Percent 0 read by radiologists blinded to cells earlier than the in- ≤ 39 Years > 39 Years the hiv status and age of the trathymic T-progenitor cell study volunteer scanned. Each Source: McCune, 1998. Reprinted with permission of the Journal of Clinical Investigation and stage, despite the depletion of thymus was rated on a “thymic the American Society for Clinical Investigation. such cells in infected grafts. index” scale of one to four, with Proviral genomes were also a score of one indicating com- not detected in colonies derived from prog- The Thymus and hiv plete involution and fat replacement and a enitor cells from infected grafts. “These while data regarding the mechanism(s) score of four indicating the presence of a data,” Dr. Jenkins’ writes in a report pub- of hiv-mediated thymus destruction in hu- large amount of thymic tissue. In analyz- lished in Blood, “demonstrate that hiv in- mans are limited, there are a number of ing the results, study participants were fection interrupts both lineage-restricted theoretical possibilities based on what is divided into two groups: those with a score and multilineage hematopoiesis in vivo known about the effects of hiv on human T- of three or four were said to have “abun- and suggest that depletion of early cell subpopulations (Hellerstein, 1997). For dant” thymic tissue while those with a hematopoietic progenitor cells occurs in starters, early depletion of naive (CD45RA+ score of one or two were said to have the absence of direct viral infection.” CD62L+) CD4+ and CD8+ cells is a fundamen- “non-abundant” thymic tissue. The fact that hiv does not infect tal feature of hiv infection. Since these cells Dr. McCune’s team hypothesized that, hematopoietic progenitor cells may prove to are relatively resistant to productive infec- if the thymic index truly reflects a func- be a fruitful finding. As is discussed in our tion of hiv—because they do not divide tional organ making CD4+ and CD8+ cells, an lymphoma review, beginning on page 22, rapidly, if at all—it is possible that they are abundant organ seen by ct should correlate Dr. David Scadden and his colleagues have depleted by viral-induced destruction of with a higher number of CD45RA+ CD62L+ been looking at the possibility of tweaking bone marrow- and thymus-derived pro- (naive) T-cells in the peripheral blood. This hematopoietic progenitor cells to produce genitor cells, which was demonstrated in correlation held: a thymic index of three or hiv-resistant T-lymphocytes progenies, per- the Jenkins paper cited above. Second, in- four—reported in 47/99 (47%) of the hiv-in- haps as an adjunctive component of bone- trauterine hiv infection of the thymus can fected adults, aged 20 to 59—was signifi- marrow transplantation in hiv-related lym- lead to thymic aplasia in children, resulting cantly associated with both higher CD4+ phoma cases. in thymic failure, loss of naive CD4+ and cell counts and a higher percentage and As for thymopoietic progenitor cells, CD8+ cells in peripheral circulation and absolute number of circulating naive CD4+ Dr. McCune explained that, despite the rapid progression to aids and death. Like- cells. The prevalence of abundant thymus protective nature of its vascular endothe- wise, older age at the time of hiv infection was especially high, as expected, in younger lium, the thymus and its cells are a highly is one of the most important factors of dis- hiv-positive adults, regardless of their CD4+ vulnerable target for hiv infection: greater ease progression rates in adults, a correla- cell count. But, while the thymus appeared than 90% of all thymocytes carry the CD4 tion that may be related to the fact that to become less abundant in hiv-negative receptor and many of these cells also ex- thymic reserves are diminished with age. study volunteers as they got older—no press the co-receptor CXCR4 and/or CCR5 Third, the T-cell receptor (tcr) repertoire of seronegative person past the age of 39 had (Berkowitz, 1998). Evidence of hiv infection both CD4+ and CD8+ cells is often restricted an abundant thymus—the same situation of the thymus can be found in numerous in advanced hiv disease, indicating that did not hold for the hiv-positive group: reports indicating morphologic changes in the CD4+ cell pool has expanded from a many of these individuals showed evidence pediatric and adult thymus specimens, im- limited number of tcr clonotypes. This ob- of abundant thymus even if they were over munohistochemical visualization in thy- servation, in turn, suggests that the thy- the age of 39 (see Figure 2). Strikingly, mocytes, and isolation of hiv from the or- mus is no longer able to generate a diverse younger (39 years or less) hiv-positive sub- gan (McCune, 1997). T-cell receptor repertoire. jects with a CD4+ count in the range of 300- While the above observations predict 500 cells/mm3 showed ct evidence of abun- that hiv-infected adults should have little dant thymus. “Contrary to our preconceived if any thymic function, a study conducted notions going into this study, we had a by Dr. McCune and his group instead came large number of hiv-positive patients with

THE PRN NOTEBOOK™ • VOLUME 7, NUMBER 3 • SEPTEMBER 2002 • WWW.PRN.ORG 7 healthy thymic tissue,” Dr. McCune stated in tissues. This was characterized by an in- lated declines in thymopoiesis and accel- wrapping up his discussion of this study. “In creased percentage of IL-7-positive cells erates immune reconstitution in immun- such patients, we would expect haart to and increased amounts of IL-7 produced odeficient animals. Although growth hor- permit thymic production of naive T-cells.” per cell in biopsied lymph nodes. mone-deficient humans do not appear to Dr. McCune also reviewed IL-7 unpub- be immunologically compromised, Drs. lished data presented at the 2001 Key- Napolitano, McCune, and the rest of their stone Symposium, which were originally team hypothesized from these observa- Potential Roles for Interleukin-7 or presented by Dr. Napolitano and Dr. Ruth tions in animal models that treatment with Human Growth Hormone? Greenblatt of the Women’s Interagency hiv recombinant human growth hormone while it is still too early to explain Study (wihs). With pre- and post-haart (rhgh) would stimulate thymopoiesis in why some hiv-positive adults have abun- samples available from 237 hiv-positive HIV-1-infected individuals. dant thymuses, it can possibly be attrib- women, Dr. Napolitano’s group confirmed The study, which was published in a uted to the increased secretion of a positive its earlier results by demonstrating a strong May issue of AIDS and discussed further at regulator, such as interleukin-7 (IL-7), and independent correlation between cir- the XIV International aids Conference in growth hormone, stem-cell factor, Flt3L, or culating IL-7 levels and lymphopenia. Of Barcelona, involved five hiv-infected adults IL-2. In other words, just as a loss of red particular interest, pre-haart IL-7 levels treated with rhgh (Serostim) 3 mg/day for blood cells or platelets is countered with predicted the degree of CD4+ cell recovery six to 12 months in a prospective fashion the production of erythropoietin or throm- upon initiating haart, and post-haart IL-7 (Napolitano, 2002; 2002a). The dose was bopoietin to stimulate the proliferation of levels decreased as the CD4+ cells recov- reduced to 1.5 mg per day after the first 6 early erythroid or megakaryocytic pro- ered. “Here we were able to suggest that months of therapy, except in one subject, in genitors, hiv-induced depletion of CD4+ increased IL-7 levels may function to in- whom gh dose reduction occurred at month cells might elicit a factor that stimulates increase lymphocyte production and/or ex- 2 as a result of persistent arthralgias. Im- creased (or renewed) production of CD4+ pansion,” Dr. McCune commented. “These munological analyses were performed be- cells in the thymus and elsewhere. data support a homeostatic role for IL-7. fore rhgh treatment and repeated at three- Among these positive regulators of However, the association between IL-7 lev- month intervals after rhgh initiation. thymopoiesis, Dr. McCune pointed out that els and immune recovery may not be evi- Thymic mass was analysed using comput- IL-7 has been shown to play a pivotal role. dent in individuals with irreversible hiv- ed tomography with quantitative density In IL-7 knockout mice, very few T-cells are mediated destruction of thymus, bone mar- and volume analysis. Analysis of circulating found in the thymus and peripheral lym- row, or lymph nodes.” lymphocytes, including naive and memory phoid organs. He noted that receptors for While it will be important to continue T cell subsets, was also performed using IL-7 are found on lineage-restricted pro- pondering the role of IL-7 on thymopoeisis multiparameter flow cytometry. genitors in lymph node, thymus, and bone in hiv-positive patients, its potential as a There was a marked increase in thymic marrow. “We also know that elevated IL-7 therapeutic compound has barely begun to tissue in all subjects after six months of levels are detected in non-hiv lymphopenic be established. “First off,” Dr. McCune rhgh therapy. At baseline, the five patients conditions, such as severe combined im- said, “IL-7 is not yet available for clinical had thymic atrophy as evidenced by the mune deficiency syndrome and acute lym- studies. It may also pose problems if ad- near-complete replacement of the thymus phocytic leukemia—and these levels usu- ministered as a therapy for hiv patients. IL- by fat on ct scan (mean thymic index of ally normalize when the lymphopenia re- 7 can increase hiv replication and, by stim- one). Repeat analysis after six months of solves,” explained Dr. McCune. ulating the usually dormant thymus, may treatment revealed a prominent increase in First up, then, was to test the hypoth- open it up to infection. However, IL-7 might dense thymus tissue in all of the rhgh re- esis that hiv-mediated CD4+ cell loss in- be safely given to promote thymopoiesis if cipients (mean thymic index of four, P = duces the production of factors that are ca- viral replication is controlled using haart. 0.0002 compared with baseline). Quanti- pable of stimulating lymphocyte develop- In addition, IL-7 could be useful in non- tative density and volume measurements at ment and expansion. Drs. Laura Napoli- hiv-infected patients with severe immune six months demonstrated a mean increase tano, McCune, and their group at the Glad- deficiency, such as in the setting of in thymic density of 86 hu (P = 0.0008), stone Institute performed cross-sectional (n chemotherapy or bone marrow trans- and a mean increase in thymic volume of = 168) and longitudinal (n = 11) analyses plantation.” 88% (P = 0.06). To determine whether the showing that increased circulating levels of With respect to growth hormone, Dr. reversal of thymic atrophy was caused by interleukin (IL)-7 were strongly associated McCune explained that animal studies a generalized lipolytic effect of rhgh, quan- with CD4+ lymphopenia in hiv disease have indicated that it plays an important titative density analysis was performed on (Napolitano, 2001). Using immunohisto- role in mammalian thymopoiesis. Growth the axillary adipose tissue of each patient. chemistry with quantitative image analy- hormone may act directly upon immune No increase in axillary adipose density sis, Dr. Napolitano—working with Dr. Jan tissues or its effects may be mediated in- was detected (mean change of +3 hu at Andersson of the Karolinska Institute in directly through insulin-like growth factor- six months and –9 hu at one year). Stockholm—also demonstrated that IL-7 1 (IGF-1). Rodents deficient in growth hor- Treatment with rhgh was also associat- is produced by dendritic-like cells within mone exhibit thymic hypoplasia that im- ed with an increase in the percentage and peripheral lymphoid tissues and that IL-7 proves with growth hormone replacement. absolute number of naive CD4+ cells. When production by these cells was significant- In older rodents, the administration of compared with baseline values, the mean ly increased within lymphocyte-depleted growth hormone or IGF-1 reverses age-re- absolute gain in the naive CD4+ cell per-

8 THE PRN NOTEBOOK™ • VOLUME 7, NUMBER 3 • SEPTEMBER 2002 • WWW.PRN.ORG centage was 6% at six months, 10% at nine References months, and 12% at 12 months; both the Berkowitz RD, Beckerman KP, Schall TJ, et nine-month and 12-month increases were al. CXCR4 and CCR5 expression delineates targets for HIV-1 disruption of T-cell differ- statistically significant. Naive CD8+ cells did entiation. J Immunol 161:3702-10, 1998. not increase with rhgh treatment, and no significant changes in naive CD4+ cell per- Haase AT. Population biology of HIV-1 infec- centages or naive CD8+ cell percentages tion: viral and CD4+ T cell demographics and dynamics in lymphatic tissues. Annu were seen in historical control subjects who Rev Immunol 17:625-656, 1999. were maintained on effective antiretroviral therapy for a similar period of time. Hellerstein M, Hanley M, Cesar D, et al. Di- To date, two study participants have rectly measured kinetics of circulating T been off rhgh therapy for one or more lymphocytes in normal and HIV-1-infected humans. Nat Med 5:83-9, 1999. years. Repeat thymus ct scans, performed after therapy discontinuation, revealed a Hellerstein M and McCune J. T-cell turnover decrease to baseline thymic density in in HIV-1 disease. Immun 7:583-9, 1997. both individuals. However, gains in naive Jenkins M, Hanley MB, Moreno MB, et al. Hu- CD4+ cells remained stable. man immunodeficiency virus-1 infection in- It goes without saying that these find- terrupts thymopoiesis and multilineage ings do not support the general use of hematopoiesis in vivo. Blood 91(8):2672-8, rhgh with the intent of reversing immune 1998. deficiency. Certain limitations of this study, McCune JM. The dynamics of CD4+ T-cell de- including the small number of treated sub- pletion in HIV disease. Nature 410:974-9, 2001. jects and the lack of a randomized control arm, require that these data be interpret- McCune JM, Loftus R, Schmidt DK, et al. High ed with caution. Fortunately, a larger, ran- prevalence of thymic tissue in adults with human immunodeficiency virus-1 infection. domized study is currently under way to J Clin Invest 101(11):2301-8, 1998. evaluate further the role of rhgh as an immune-based therapy. McCune JM. Thymic function in HIV-1 disease. Semin Immunol 9(6):397-404, 1997.

Napolitano LA, Lo JC, Gotway MB, et al. In- creased thymic mass and circulating naive Summary CD4 T cells in HIV-1-infected adults treated in an effort to summarize the rather with growth hormone. AIDS 16(8):1103-11, complex components of his talk, Dr. Mc- 2002. Cune stressed that little doubt remains Napolitano LA, Chien AJ, Hanley MB, et al. En- regarding the destructive nature of hiv on hancement of human T-lymphopoiesis by mature effector CD4+ cells in the human growth hormone [Abstract ThOrA1484]. XIV body. However, it is equally important to International aids Conference, Barcelona, 2002. remember that compensatory feedback Napolitano LA, Grant RM, Deeks SG, et al. In- pathways likely help to sustain the CD4+ creased production of IL-7 accompanies HIV- cell count in the face of destruction. When 1-mediated T-cell depletion: implications these pathways fail, regenerative failure is for T-cell homeostasis. Nat Med 7(1):73-9, not far behind. One possible mechanism is 2001. the hiv-mediated depletion of long-lived progenitor cells, including those in the bone marrow, the thymus, and in peripheral lymphoid organs. Another possible mechanism is the loss of feedback loops, such as those mediated by IL-7 and other positive regulators, known to play a role in hematopoiesis and thymopoiesis. “More focus on regenerative failure may teach us about the hard-wiring of human T-cell homeostasis,” Dr. McCune said. “At the same time, it may provide additional clues for adjunctive therapies, such as IL-7 and recombinant human growth hormone, for T-cell reconstitution.”

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