Thymic volume predicts long-term immune reconstitution in HIV-infected children treated with highly active antiretroviral therapy

Highly active antiretroviral therapy (HAART) suppresses viral replication and augments CD4 T cell counts. HAART-associated immune restoration is often difficult to predict. We verified whether increases in CD4 cells, and particularly in cells of the naive phenotype, would be associated in HAART-treated children with thymic volume. Long-term immune reconstitution is significantly better in children with bigger thymuses at the initiation of HAART. Thymic volume has a strong predictive value for the immunological effect of HAART. Highly active antiretroviral therapy (HAART) suppresses viral replication and augments CD4 T cells in HIV-infected individuals. The virological effect of HAART is highly reproducible; in contrast, the degree of immune restoration is variable among different patients and is difficult to predict [1,2]. CD4 T cells include both memory (CD4+CD45RO+) and naive (CD4+CD45RA+CD62L+) subpopulations. In HAART-treated adults, increases in CD4 T cells are for a long period of time supported by the redistribution of memory lymphocytes, whereas early increases in naive lymphocytes are seen in paediatric patients [2,3]. Because thymic maturation and selection lead to the generation of naive T cells, we verified whether the degree of HAART-associated immune reconstitution was associated with thymic volume. We report here that immune reconstitution after 3 years of therapy is significantly better in children with bigger thymuses at the initiation of HAART. Thymic volume thus has a strong predictive value for the immunological effect of HAART. Thymus evaluation was performed using magnetic resonance imaging in 20 HIV vertically infected children. Using this technology, thymic tissue appears sharply hypointense in comparison with mediastinal fat, and is easily distinguishable from vessels. Sagittal and axial slices (5 mm) were performed with a 1.5 T magnet with fast spin echo and T1 and T2 weighted acquisitions. Thymic dimensions were calculated according to the formula: 3.14/6 × D1 × D2 × D3 using the transverse diameter of the gland (D1) on the axial images and the antero-posterior (D2) and cranio-caudal (D3) diameters obtained from sagittal sections. The median thymic volume at baseline was 10.39 cm3; this value was used to divide children into those with a thymus bigger or smaller than the median. The mean age (10.6 versus 9.8 years) was comparable between the two groups of patients, who all had moderate-to-advanced disease according to the 1994 revised Centers for Disease Control and Prevention HIV classification. Children had previously been treated with zidovudine alone (eight patients) or in association with didanosine (12 patients), and were subsequently switched to HAART [lamivudine (4 mg/kg per 12 h) plus stavudine (1 mg/kg per 12 h) plus indinavir (500 mg/m2 per 8 h)]. At baseline, HIV plasma viraemia (21 570 versus 18 758 copies/ml) was comparable in children with thymic volumes above or below the median. Absolute CD4 cell counts and the percentage of naive CD4 T lymphocytes were moderately higher in children with bigger thymuses (Fig. 1), whereas the percentage of CD4 T cells was similar between the two groups of patients.Fig. 1.: CD4 cell absolute numbers (a,b), CD4 cell percentages (c,d), and percentages of CD4+RA+62L+ (naive) lymphocytes (e,f) in 20 HIV-infected children studied before the beginning of highly active antiretroviral therapy (baseline) and after 36 months of therapy. Panels a, c and e show values observed in children whose thymic volume determined by magnetic resonance imaging at baseline was above the median value of the group; panels b, d and f show values observed in children whose thymic volume was below the median value. Each symbol represents a different child. Mean values and statistical differences are shown.Immunological and virological parameters were re-evaluated after 36 months of therapy. HAART was associated with a dramatic reduction in plasma viraemia in both groups of children (plasma viraemia < 80 RNA copies/ml in 19/20 individuals), whereas the effect of antiviral therapy on immune parameters was more conspicuous in children with thymic volumes above the median at baseline. HAART-associated increases in both CD4 cell counts and CD4 cell percentages were significantly more impressive in these children (Fig. 1a and c) compared with those with smaller thymuses at baseline (Fig. 1b and d). As a consequence, the median CD4 : CD8 cell ratio increased from 0.48 to 1.19 (p < 0.0001) in children with bigger thymuses and only from 0.34 to 0.57 (not significant) in children with smaller thymuses. Naive CD4 T lymphocytes drastically increased in children with bigger thymuses (Fig. 1e), and were only marginally modified in patients with smaller thymuses (Fig. 1f). Finally, HAART resulted in an augmentation of the thymus, which, again, was more evident when children with bigger thymuses (median 12.41 versus 21.81 cm3; P = 0.0005) were compared with those with thymuses below the median at baseline (median 8.21 versus 10.73 cm3; P = 0.03). HAART-associated suppression of HIV replication is very similar in different patients. In contrast, the degree of HAART-associated immune reconstitution is variable among individuals. No established marker predicts the degree of immune reconstitution in HAART-treated patients. In particular, immune reconstitution in children is independent of age and pre-therapy immune status [1], and has been suggested, but not proved, to be correlated with thymic function [4–6]. We studied the immunovirological effect of HAART in 20 HIV-infected children of similar age and comparable viraemia who underwent the same therapeutic regimen, and observed that, as expected, although HAART resulted in the suppression of virus replication in all children but one, increases in CD4 cell counts were highly variable. We report here that the size of thymus at baseline predicts the degree of long-term immune reconstitution of HAART, independent of the virological effects of therapy. This observation is novel, and expands on a previous report in adults [7], which suggested that thymus volume, measured after 48 weeks of therapy, could be associated with a larger increase in the pool of circulating CD4 T cells. Naive T cells are thymic efferent. In adults, in whom physiological thymic regression has taken place, these cells could be generated in thymic residues, in extrathymic compartments, or can stem from CD45RO cells of the CD45RA phenotype [3]. Our data confirm that, similar to what has been shown after intensive chemotherapy or bone marrow transplant [8], the thymic pathway is of pivotal importance in the re-population of peripheral blood with T lymphocytes in HIV-infected children undergoing HAART. More aggressive antiviral regimens or the use of immune modulators might be considered in patients with smaller thymuses.