Background: Emerging evidence indicates that CD4 and CD8 T cell recovery is differentially regulated during HIV infection. The hallmark of interleukin-2 (IL-2)-induced immune reconstitution is the selective outgrowth of CD4 through undefined mechanisms.
Objective: To delineate the effect of IL-2 on T cell homeostasis by analysing the differential impact of IL-2 immunotherapy on CD4 and CD8 dynamics.
Design: A randomized trial of 15 HIV-positive patients, eight receiving IL-2 immunotherapy with highly active antiretroviral therapy (HAART) and seven with HAART alone. Patients were followed for a 48-week period following three IL-2 cycles (overall, 10 weeks in duration).
Methods: CD4 and CD8 count, naive and memory immunophenotype, proliferation by Ki67, and CD8+CD38+ activated pattern were measured longitudinally by flow cytometry. Thymic output contribution to both CD4 and CD8 was evaluated by measurement of T cell receptor excision circles (TREC). Wilcoxon test was used to compare results.
Results: Compared with changes seen with HAART alone, IL-2 induced a more significant rise in CD4 than CD8 T cell count (P < 0.01), associated with a significant increase in Ki67-proliferating CD4 (P < 0.05), whereas no changes were seen in CD8+Ki67+ (P > 0.05). Furthermore, IL-2 administration was associated with CD4 TREC increase, whereas CD8 TREC remained stable (P > 0.05). Modifications in CD4 and CD8 T cells seen in patients taking only HAART were not associated with changes in CD4 and CD8 TREC.
Conclusions: By showing a differential impact on CD4 and CD8 homeostasis, the study suggests that IL-2-associated immune reconstitution results from protean interactions between T cell compartments; this has significant implications for the correct planning of immunotherapeutic strategies.
From the aInstitute of Infectious Diseases and Tropical Medicine, ‘Luigi Sacco’ Hospital and the bDepartment of Immunology, University of Milan, Milan, Italy.
Correspondence to Dr G. Marchetti, Institute of Infectious Diseases and Tropical Medicine, ‘Luigi Sacco’ Hospital, University of Milan, Via G.B. Grassi, 74-20157 Milan, Italy.
Received: 25 February 2003; revised: 23 June 2003; accepted: 17 July 2003.
HIV-1 infection exerts a differential impact on CD4 and CD8 T cells, causing fundamental changes in the lifecycle of CD4 but not of CD8 T cells [1–4]. This ultimately results in the preferential depletion of the CD4 cell compartment whereas the CD8 T cells keep expanding until late in the course of infection.
Immune reconstitution following highly active antiretroviral therapy (HAART) is characterized by distinct CD4 and CD8 T cell dynamics, often displaying dichotomist trends according to disease stage [5,6]. Therefore, it is intriguing to speculate that CD4 and CD8 T cells posses inherent biological differences in their peculiar regenerative dynamics , which could become manifest under both endogenous and exogenous perturbations of physiological homeostasis [8–10].
Interleukin-2 (IL-2) is the main T cell growth factor, leading to proliferation and differentiation of the whole T lymphocyte compartment. However, the hallmark of immune reconstitution following low-dose IL-2 adjuvant therapy in HIV-positive patients is the selective expansion of CD4 T cells [11–13], ultimately resulting in the relative outgrowth of this T cell subset.
To acquire deeper insight into the effect of IL-2 immunotherapy on T cell homeostasis, the reconstitution dynamics of CD4 and CD8 T cells were compared in patients treated with IL-2 plus HAART and those treated with HAART alone (controls), with particular focus on cell proliferation and neothymic output. A better understanding of the contribution of cell turnover and neothymopoiesis to the shaping of the CD4 and CD8 IL-2-driven rescue will help to delineate the interplay between T lymphocyte subsets in HIV infection, thus providing an innovative framework for the most targeted and clinically advantageous immune-based strategies.
The trial included 15 HIV-positive immunological non-responders with HIV RNA < 50 copies/ml and CD4 T cell count constantly ≤ 200 × 106 cells/l after at least 12 months of stable HAART. The study was approved by the Ethical Committee of ‘Luigi Sacco’ Hospital, Milan, Italy. Eight patients received IL-2 and HAART and seven received HAART only. After three low-dose cycles of IL-2 (one cycle included 3 × 106 IU daily subcutaneously at days 1–5 and 8–12), for an overall duration of 10 weeks, all the patients were followed over 48 weeks . CD4, CD8, naive CD4+45RA+62L+, memory CD4+45RA− and activated CD8+CD38+ T lymphocytes were determined longitudinally by flow cytometry (Elite ESP, Coulter, Florida, USA) using CD4–peridinin chlorophyll protein, CD8–fluorescein isothiocyanate (FITC), CD45RA–FITC, CD62L–phycoerithrin (PE) and CD38–PE. After immunomagnetic sorting of CD4 and CD8 T cells from peripheral blood mononuclear cells (purity > 95%; CD4 and CD8 Positive isolation kit; Oxoid, Milan, Italy), neothymopoiesis was evaluated by measuring the VαJα coding joint in T cell receptor excision circles (TREC) with a polymerase chain reaction enzyme-linked immunosorbent assay . In order to minimize the dilution effect of cell turnover on TREC frequency, total TREC (expressed as TREC/μl blood) was calculated as (TREC/CD4 T cells) × (absolute number CD4 T cells). To take the effect of cell turnover into further account, cellular proliferation rate was evaluated by flow cytometric measurement of the expression of Ki67 nuclear antigen (clone MIB-1, Immunotech, Westbrook, Maine, USA) on purified CD4 and CD8 T cells, as previously described . All the parameters were analysed at four time points: before study initiation (week 0), at the end of IL-2 administration (week 10) and after 34 (week 34) and 48 weeks (week 48). Statistical analysis was performed by Wilcoxon signed rank test.
Figure 1 shows the schedule of dosing with low- dose prolonged intermittent IL-2 immunotherapy and HAART. There were no baseline differences between the eight patients receiving IL-2 and the seven receiving only HAART in clinical–epidemiological features , CD4 and CD8 counts, turnover and TREC (Fig. 2). In particular, both groups had comparable median age: IL-2 patients 36 years (range, 30–55) and HAART controls 37 years (range, 26–40).
Table 1 shows CD4/CD8 ratio, CD8+CD38+ activated T lymphocytes, naive and memory immunophenotype, in IL-2-receiving patients and in HAART-alone controls during the whole study period.
IL-2 immunotherapy resulted in a significant and sustained rise in both absolute and percentage CD4 T cells (weeks 10 and 48; P < 0.01) and CD4/CD8 cell ratio (weeks 10 and 48; P < 0.05), whereas CD8 count remained stable up to 48 weeks (P = 0.05) (Table 1; Fig. 2a–d). At week 48, CD4 T cells displayed a more relevant recovery than CD8 T cells in percentage terms (P < 0.05), with a tendency toward higher rescue also in absolute count (P = 0.069) (Fig. 2a–d). Conversely, HAART controls displayed a constrained yet parallel CD4 and CD8 recovery, reaching significance only in absolute value at 48 weeks (CD4 cells P < 0.01; CD8 cells P < 0.05), with no significant changes in CD4/CD8 ratio at all time points (P > 0.05) (Fig. 2a–d). At week 48, IL-2 patients experienced significantly higher recovery than the HAART-alone controls in CD4 cells (P < 0.01), but not in CD8 cells (P > 0.05) (Fig. 2a–d).
Since peripheral T cell pool is maintained by both proliferation and renewal, the contribution of cell turnover and neothymopoiesis to both subsets was examined. At baseline, overall Ki67 expression was significantly higher in CD8 cells (4%) than in CD4 cells (1.6%) (P < 0.01), confirming data describing relatively higher turnover rates within the CD8 subset [15,16] as a possible on-going response to low-level viral replication despite successful HAART [17,18]. IL-2 induced a significant rise in Ki67+CD4+ at week 10, from 2.1% to 3.4% (P < 0.05), but not in Ki67+CD8+ (P > 0.05). At IL-2 discontinuation, both CD4 and CD8 T cell turnover progressively dropped to below pretreatment values (Fig. 2e) . However, in HAART-alone controls, the frequency of proliferating CD4 and CD8 were closely matched at all time points (Fig. 2f).
To evaluate the role played by IL-2 on CD8 T cell dynamics further, the CD8 T cell turnover data was integrated by studying the CD8+CD38+ (activated) lymphocytes . At baseline, IL-2-receiving patients and controls displayed comparable proportions (P > 0.05) (Table 1). Similar to the effect on CD8 cell turnover, IL-2 did not induce an elevation in the percentage of CD8+CD38+ T lymphocytes (week 10; P > 0.05), which indeed progressively decreased, reaching significance by week 34 (week 34 P < 0.05; week 48 P < 0.01) [21,22]; no changes were observed in the HAART controls (week 48; P > 0.05) (Table 1).
Analysis of individual CD4 subsets showed a discrepant pattern of recovery between IL-2-patients and HAART-alone controls. In particular, IL-2 immunotherapy induced a significant and sustained expansion of both CD4+CD45RA+CD62L+ (naive) (week 10 and 48; P < 0.05) and CD4+CD45RA− (memory) (week 10 and 48; P < 0.05) subsets (Table 1), whereas HAART alone resulted in a preferential rescue of memory CD4 cells (week 48; P < 0.05) with no significant changes in the naive compartment (week 48; P > 0.05) (Table 1). At week 48, IL-2 patients experienced higher naive and memory CD4 recovery compared with controls (P < 0.01) (Table 1).
Having demonstrated that IL-2 immunotherapy induces a preferential expansion of CD4 T cells [11-13], a large proportion of which bear a naive phenotype, the impact of neothymic output on both CD4 and CD8 subsets was analysed. As with other parameters, CD4 and CD8 T cells displayed a dichotomy in TREC content, with a temporary tendency to increase at week 10 in CD4 cells (from 4.1 × 106 to 6.5 × 106 copies/μl; P = 0.08) and no changes within CD8 pool (P > 0.05) (Fig. 2g). As TREC signal is affected by proliferative dilution [23,24], the concurrent rise in CD4 TREC and turnover strongly suggests a more substantial actual increase in CD4 neothymopoiesis. No significant CD4 and CD8 TREC changes were seen in HAART controls at all time points (P > 0.05) (Fig. 2h). Overall, at week 10, IL-2-receiving patients experienced a more relevant CD4 TREC increase than did HAART controls (P < 0.05), whereas no significant differences were shown in CD8 TREC (P > 0.05) (Fig. 2g,h).
This study investigated the long-term kinetics of CD4 and CD8 T cells in a cohort of HIV-positive patients treated with low-dose prolonged intermittent IL-2 immunotherapy compared with patients receiving HAART alone. The results show different IL-2-driven dynamics in CD4 and CD8 subsets, clearly demonstrating that IL-2 has a differential impact on these T lymphocyte subsets, with a preferential action on CD4 T cells. In particular, we confirm previous data pointing to enhanced proliferation of existing cells as the mechanism responsible for the IL-2-driven CD4 reconstitution [25,26]. Moreover, our data suggest a possible temporary boost of neothymopoiesis: the actual increase may be masked somewhat here by the increased proliferation. The disproportionate rise of proliferating CD4 versus CD8 cells  could be explained by both in vitro higher expression of IL-2 receptor mRNA in CD4 than in CD8 T cells upon activation , and the more significant in vivo IL-2-driven upregulation of IL-2 receptor α-chain (CD25) on CD4 [28,29]. By showing a preferential impact of IL-2 on CD4 neothymic synthesis, our data add to the evidence that successful CD4 reconstitution is dependent upon neothymopoiesis [7,8] and suggests a possible role of IL-2 in selectively reconstituting T helper activity to neoantigens , given that TREC increases have indeed been correlated with improved neoantigen immune function . Moreover, our data show that IL-2 does not affect CD8 cell turnover and thymopoiesis, thus implying that CD8 T cells follow different homeostatic dynamics [7,8], possibly peripheral redistribution. The broad examination of T lymphocyte dynamics during IL-2 immunotherapy provides a more detailed definition of the protean interactions between specific T lymphocyte subsets during immune reconstitution; this will be of great value in the design of immune-based strategies in the treatment of HIV infection.
We are grateful to Shane Fodgen, Greta Di Felice and Anna Cristina Sacchetta for critical reading of the manuscript and valuable advice; Maura Mezzetti (Istituto dei Metodi Quantitativi, Università ‘Luigi Bocconi', Milan) for assistance with statistical analysis; Chiron Italia for valuable advice; Bianca Ghisi for excellent typing assistance; all patients participating in the study; and Patrizia Franza, Maria Tomminello and all the staff at the Institute of Infectious Diseases and Tropical Medicine, ‘L. Sacco’ Hospital, who cared for the patients.
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