Safety, pharmacokinetics and neutralization of the broadly neutralizing HIV-1 human monoclonal antibody VRC01 in healthy adults.

VRC-HIVMAB060-00-AB (VRC01) is a broadly neutralizing HIV-1 monoclonal antibody (mAb) isolated from the B cells of an HIV-infected patient. It is directed against the HIV-1 CD4 binding site and is capable of potently neutralizing the majority of diverse HIV-1 strains. This Phase I dose-escalation study in healthy adults was conducted at the National Institutes of Health (NIH) Clinical Center (Bethesda, MD, USA). Primary objectives were the safety, tolerability and pharmacokinetics (PK) of VRC01 intravenous (i.v.) infusion at 5, 20 or 40 mg/kg, given either once (20 mg/kg) or twice 28 days apart (all doses), and of subcutaneous (s.c.) delivery at 5 mg/kg compared to s.c. placebo given twice, 28 days apart. Cumulatively, 28 subjects received 43 VRC01 and nine received placebo administrations. There were no serious adverse events or dose-limiting toxicities. Mean 28-day serum trough concentrations after the first infusion were 35 and 57 µg/ml for groups infused with 20 mg/kg (n = 8) and 40 mg/kg (n = 5) doses, respectively. Mean 28-day trough concentrations after the second infusion were 56 and 89 µg/ml for the same two doses. Over the 5-40 mg/kg i.v. dose range (n = 18), the clearance was 0.016 l/h and terminal half-life was 15 days. After infusion VRC01 retained expected neutralizing activity in serum, and anti-VRC01 antibody responses were not detected. The human monoclonal antibody (mAb) VRC01 was well tolerated when delivered i.v. or s.c. The mAb demonstrated expected half-life and pharmacokinetics for a human immunoglobulin G. The safety and PK results support and inform VRC01 dosing schedules for planning HIV-1 prevention efficacy studies.

Influenza virus h5 DNA vaccination is immunogenic by intramuscular and intradermal routes in humans.

Avian influenza virus causes outbreaks in domestic and wild birds around the world, and sporadic human infections have been reported. A DNA vaccine encoding hemagglutinin (HA) protein from the A/Indonesia/5/05 (H5N1) strain was initially tested in two randomized phase I clinical studies. Vaccine Research Center study 304 (VRC 304) was a double-blinded study with 45 subjects randomized to placebo, 1 mg of vaccine, or 4 mg of vaccine treatment groups (n = 15/group) by intramuscular (i.m.) Biojector injection. VRC 305 was an open-label study to evaluate route, with 44 subjects randomized to intradermal (i.d.) injections of 0.5 mg by needle/syringe or by Biojector or 1 mg delivered as two 0.5-mg Biojector injections in the same deltoid or as 0.5 mg in each deltoid (n = 11/group). Injections were administered at weeks 0, 4, and 8 in both studies. Antibody responses to H5 were assessed by hemagglutination inhibition (HAI) assay, enzyme-linked immunosorbent assay (ELISA), and neutralization assay, and the H5 T cell responses were assessed by enzyme-linked immunospot and intracellular cytokine staining assays. There were no vaccine-related serious adverse events, and the vaccine was well tolerated in all groups. At 1 mg, i.d. vaccination compared to i.m. vaccination induced a greater frequency and magnitude of response by ELISA, but there were no significant differences in the frequency or magnitude of response between the i.d. and i.m. routes in the HAI or neutralization assays. T cell responses were more common in subjects who received the 1- or 4-mg dose i.m. These studies demonstrated that the DNA vaccine encoding H5 is safe and immunogenic and served to define the proper dose and route for further studies. The i.d. injection route did not offer a significant advantage over the i.m. route, and no difference was detected by delivery to one site versus splitting the dose between two sites for i.d. vaccine administration. The 4-mg dose (i.m) was further investigated in prime-boost regimens.

A replication defective recombinant Ad5 vaccine expressing Ebola virus GP is safe and immunogenic in healthy adults.

Ebola virus causes irregular outbreaks of severe hemorrhagic fever in equatorial Africa. Case mortality remains high; there is no effective treatment and outbreaks are sporadic and unpredictable. Studies of Ebola virus vaccine platforms in non-human primates have established that the induction of protective immunity is possible and safety and human immunogenicity has been demonstrated in a previous Phase I clinical trial of a 1st generation Ebola DNA vaccine. We now report the safety and immunogenicity of a recombinant adenovirus serotype 5 (rAd5) vaccine encoding the envelope glycoprotein (GP) from the Zaire and Sudan Ebola virus species, in a randomized, placebo-controlled, double-blinded, dose escalation, Phase I human study. Thirty-one healthy adults received vaccine at 2×10(9) (n=12), or 2×10(10) (n=11) viral particles or placebo (n=8) as an intramuscular injection. Antibody responses were assessed by ELISA and neutralizing assays; and T cell responses were assessed by ELISpot and intracellular cytokine staining assays. This recombinant Ebola virus vaccine was safe and subjects developed antigen specific humoral and cellular immune responses.

Expansion of activated human naïve T-cells precedes effector function.

Naïve T-cells divide and mature, both functionally and phenotypically, upon stimulation through the T-cell receptor. Although much is known about the overall changes that occur in naïve cells upon TCR stimulation, and the different memory/effector populations that arise following stimulation, the relationship between cell division and functional and phenotypical changes that occur after activation is poorly understood. Here, we examine the early stages of human naïve and antigen-experienced T-cell activation, and the relationship between cell division and acquisition of effector function during the transition from resting antigen-experienced or naïve T-cells into effector cells. Stimulated naïve T-cells proliferate prior to acquisition of effector function, as measured by cytokine production and expression of effector-associated cell surface molecules. Additionally, we show that interlukin-7 (IL-7) can drive proliferation of naïve T-cells without TCR:MHC peptide interactions. IL-7 alone does not, however, drive the proliferation of antigen-experienced T-cells. Memory T-cells will divide in response to exogenous IL-7 but only in the presence of naïve T-cells and IL-2. This study contributes to the current understanding of the mechanistic differences between naïve and memory T-cell responses by defining the functional and phenotypic changes that occur to T-cells after stimulation.

Analysis of total human immunodeficiency virus (HIV)-specific CD4(+) and CD8(+) T-cell responses: relationship to viral load in untreated HIV infection.

Human immunodeficiency virus (HIV)-specific T-cell responses are thought to play a key role in viral load decline during primary infection and in determining the subsequent viral load set point. The requirements for this effect are unknown, partly because comprehensive analysis of total HIV-specific CD4(+) and CD8(+) T-cell responses to all HIV-encoded epitopes has not been accomplished. To assess these responses, we used cytokine flow cytometry and overlapping peptide pools encompassing all products of the HIV-1 genome to study total HIV-specific T-cell responses in 23 highly active antiretroviral therapy naïve HIV-infected patients. HIV-specific CD8(+) T-cell responses were detectable in all patients, ranging between 1.6 and 18.4% of total CD8(+) T cells. HIV-specific CD4(+) T-cell responses were present in 21 of 23 patients, although the responses were lower (0.2 to 2.94%). Contrary to previous reports, a positive correlation was identified between the plasma viral load and the total HIV-, Env-, and Nef-specific CD8(+) T-cell frequency. No correlation was found either between viral load and total or Gag-specific CD4(+) T-cell response or between the frequency of HIV-specific CD4(+) and CD8(+) T cells. These results suggest that overall frequencies of HIV-specific T cells are not the sole determinant of immune-mediated protection in HIV-infection.

Evidence for increased T cell turnover and decreased thymic output in HIV infection.

The effects of HIV infection upon the thymus and peripheral T cell turnover have been implicated in the pathogenesis of AIDS. In this study, we investigated whether decreased thymic output, increased T cell proliferation, or both can occur in HIV infection. We measured peripheral blood levels of TCR rearrangement excision circles (TREC) and parameters of cell proliferation, including Ki67 expression and ex vivo bromodeoxyuridine incorporation in 22 individuals with early untreated HIV disease and in 15 HIV-infected individuals undergoing temporary interruption of therapy. We found an inverse association between increased T cell proliferation with rapid viral recrudescence and a decrease in TREC levels. However, during early HIV infection, we found that CD45RO-CD27high (naive) CD4+ T cell proliferation did not increase, despite a loss of TREC within naive CD4+ T cells. A possible explanation for this is that decreased thymic output occurs in HIV-infected humans. This suggests that the loss of TREC during HIV infection can arise from a combination of increased T cell proliferation and decreased thymic output, and that both mechanisms can contribute to the perturbations in T cell homeostasis that underlie the pathogenesis of AIDS.

Poor CD4 T cell restoration after suppression of HIV-1 replication may reflect lower thymic function.

OBJECTIVE: To characterize immune phenotype and thymic function in HIV-1-infected adults with excellent virologic and poor immunologic responses to highly active antiretroviral therapy (HAART). METHODS: Cross-sectional study of patients with CD4 T cell rises of > or = 200 x 10(6) cells/l (CD4 responders; n = 10) or < 100 x 10(6) cells/l (poor responders; n = 12) in the first year of therapy. RESULTS: Poor responders were older than CD4 responders (46 versus 38 years; P < 0.01) and, before HAART, had higher CD4 cell counts (170 versus 35 x 106 cells/l; P = 0.11) and CD8 cell counts (780 versus 536 x 10(6) cells/l; P = 0.02). After a median of 160 weeks of therapy, CD4 responders had more circulating naive phenotype (CD45+CD62L+) CD4 cells (227 versus 44 x 10(6) cells/l; P = 0.001) and naive phenotype CD8 cells (487 versus 174 x 10(6) cells/l; P = 0.004) than did poor responders (after 130 weeks). Computed tomographic scans showed minimal thymic tissue in 11/12 poor responders and abundant tissue in 7/10 responders (P = 0.006). Poor responders had fewer CD4 cells containing T cell receptor excision circles (TREC) compared with CD4 responders (2.12 versus 27.5 x 10(6) cells/l; P = 0.004) and had shorter telomeres in CD4 cells (3.8 versus 5.3 kb; P = 0.05). Metabolic labeling studies with deuterated glucose indicated that the lower frequency of TREC-containing lymphocytes in poor responders was not caused by accelerated proliferation kinetics. CONCLUSION: Poor CD4 T cell increases observed in some patients with good virologic response to HAART may be caused by failure of thymic T cell production.

Monitoring HIV-specific CD8+ T cell responses by intracellular cytokine production.

Human immunodeficiency virus (HIV)-specific CD8+ T cells play an important role in controlling HIV infection. Accurate monitoring of these cells is crucial in determining the effects of HIV therapy and vaccine efficacy. Using an intracellular cytokine staining based assay, we are able to directly quantify functional HIV-specific CD8+ T cells. This assay is highly reproducible, and can be performed using both fresh and cryopreserved peripheral blood cells. Importantly, this assay can be used to examine multiple HIV-peptide epitopes simultaneously, and is independent of patient HLA haplotype. Here, we examine the HIV-specific CD8+ T cell response to 95 optimized HIV-derived cytotoxic T lymphocyte (CTL) epitopes in 21 HIV-infected patients of varying HLA haplotype, using peptide mixes and matrices. We find that when using mixes of multiple HIV peptides, the CD8+ T cell response to the mixture is equivalent to the sum of the responses to the individual peptides contained therein. Detailed comparison of the responses in patients suggests that most patients generate a diverse CD8+ T cell response, recognizing multiple HIV epitopes derived from HIV Gag, Pol, Env, or Nef. Although some patients sharing HLA alleles occasionally recognize common peptides, rarely are responses to those peptides dominant within the same group of patients. These results confirm our previous findings that the responses to single HIV-peptides are rarely representative of the entire HIV response.

Adaptation to blockade of human immunodeficiency virus type 1 entry imposed by the anti-CCR5 monoclonal antibody 2D7.

The second extracellular loop (ECL2) domain of CC-chemokine receptor 5 (CCR5) has been proposed as a specific target site for therapeutic agents aimed at blocking CCR5-dependent entry by human immunodeficiency virus type I (HIV-1). We have adapted two CCR5-using HIV-1 isolates, prototypic JR-CSF, and a primary isolate, 11-121, to replicate in vitro in the presence of high concentrations of a monoclonal antibody (MAb 2D7) specific for the CCR5 ECL2 domain. The 75% inhibitory concentrations (IC(75)) for the two 2D7-adapted isolates were approximately 100-fold higher than those for corresponding control isolates passaged without the MAb. Adapted isolates did not acquire the ability to use CXCR4, CCR3, or CCR1. Env clones derived from MAb 2D7-adapted JR-CSF showed several gp120 mutations that were not found in any of the control JR-CSF clones. The in vitro observations suggest that CCR5-using HIV-1 strains might also be able to adapt in vivo to evade an ECL2-blocking therapeutic agent.

Decay kinetics of human immunodeficiency virus-specific CD8+ T cells in peripheral blood after initiation of highly active antiretroviral therapy.

We measured the longitudinal responses to 95 HLA class I-restricted human immunodeficiency virus (HIV) epitopes and an immunodominant HLA A2-restricted cytomegalovirus (CMV) epitope in eight treatment-naive HIV-infected individuals, using intracellular cytokine staining. Patients were treated with highly active antiretroviral therapy (HAART) for a median of 78 weeks (range, 34 to 121 weeks). Seven of eight patients maintained an undetectable viral load for the duration of therapy. A rapid decline in HIV-specific CD8(+) T-cell response was observed at initiation of therapy. After an undetectable viral load was achieved, a slower decrease in HIV-specific CD8(+) T-cell response was observed that was well described by first-order kinetics. The median half-life for the rate of decay was 38.8 (20.3 to 68.0) weeks when data were expressed as percentage of peripheral CD8(+) T cells. In most cases, data were similar when expressed as the number of responding CD8(+) T cells per microliter of blood. In subjects who responded to more than one HIV epitope, rates of decline in response to the different epitopes were similar and varied by a factor of 2.2 or less. Discontinuation of treatment resulted in a rapid increase in HIV-specific CD8(+) T cells. Responses to CMV increased 1.6- and 2.8-fold within 16 weeks of initiation of HAART in two of three patients with a measurable CMV response. These data suggest that HAART quickly starts to restore CD8(+) T-cell responses to other chronic viral infections and leads to a slow decrease in HIV-specific CD8(+) T-cell response in HIV-infected patients. The slow decrease in the rate of CD8(+) T-cell response and rapid increase in response to recurrent viral replication suggest that the decrease in CD8(+) T-cell response observed represents a normal memory response to withdrawal of antigen.

RANTES production from CD4+ lymphocytes correlates with host genotype and rates of human immunodeficiency virus type 1 disease progression.

Several chemokine and chemokine receptor parameters were measured in peripheral blood mononuclear cells obtained from patients before they became infected with human immunodeficiency virus type 1 (HIV-1). After HIV-1 infection, the parameters were compared with plasma HIV-1 RNA levels and with rates of CD4(+) lymphocyte decline. Patients who were heterozygous for the Delta32CCR5 allele had significantly higher levels of RANTES production from their CD4(+) lymphocytes than did patients who did not carry the Delta32CCR5 allele (P=.01). Higher RANTES production levels from ex vivo-activated CD4(+)-enriched lymphocytes, but not CD8(+) lymphocytes, correlated with lower plasma HIV-1 RNA levels 9-12 months after infection (P= .01) and with slower rates of CD4(+) lymphocyte decline (P= .002). CCR5 expression levels on ex vivo-activated CD4(+) lymphocytes did not correlate with markers of disease progression. These results further support the hypothesis that chemokine production levels are associated with HIV-1 replication in vivo.

Effect of thymectomy on human peripheral blood T cell pools in myasthenia gravis.

The human thymus is required for establishment of the T cell pool in fetal life, but postnatal thymectomy does not lead to immunodeficiency in humans. Because thymectomy in humans is performed for treatment of myasthenia gravis (MG), we have studied patients with MG for effects of thymectomy on peripheral blood (PB) naive (CD45RA(+), CD62L(+)) and memory (CD45RO(+)) T cells. We have also determined the effect of thymectomy on levels of PB cells containing signal joint TCR delta excision circles (TRECs), a molecular marker of thymus emigrants that have divided few times after leaving the thymus. In 17 nonthymectomized and 26 thymectomized MG patients studied at varying times after thymectomy (1 day to 41 years), we found no significant mean difference in PB T cell TREC levels between ages 40 and 80 years. However, both thymectomized and nonthymectomized MG patients had lower PB T cell TREC levels than did age-matched normal subjects (p < 0.0001 for both). These data demonstrated that MG itself or treatment for MG decreased thymopoiesis independent of thymectomy. Next, to control for disease activity and treatment, we prospectively studied 10 MG patients before and from 27 to 517 days after thymectomy. We found that thymectomy decreased CD4 or CD8 T cell TREC concentrations most when thymopoiesis was active before thymectomy (six of six patients), but had little effect in patients when thymopoiesis was minimal (four of four patients). In contrast, there was no significant effect of thymectomy on absolute numbers of naive PB T cells. Thus, in MG, removal of a thymus with active thymopoiesis resulted in a significant fall in PB TREC(+) T cells postthymectomy.

Factors affecting thymic function after allogeneic hematopoietic stem cell transplantation.

Hematopoietic stem cell transplantation (HSCT) is followed by profound immunodeficiency. Thymic function is necessary for de novo generation of T cells after HSCT. Circulating CD45RA(+) naive T-cell levels are predictive of antigen-specific T-cell responses in the absence of graft-versus-host disease (GVHD). These T cells may not represent recent thymic emigrants, since naive T cells may maintain this phenotype if not antigen-activated. To accurately measure thymic output after HSCT and determine the factors that influence thymic function, T-cell receptor excision circles (TRECs) were examined in CD4(+) and CD8(+) cells from a cross-section of patients following HSCT. TREC levels rose weeks after HSCT and could be detected in patients 6 years after HSCT. TREC levels correlated with the frequency of phenotypically naive T cells, indicating that such cells were not expanded progeny of naive T cells present in the donor graft. Chronic GVHD was the most important factor that predicted low TREC levels even years after HSCT. Patients with a history of resolved GVHD had decreased numbers of TREC, compared with those with no GVHD. Because few adults had no history of GVHD, it was not possible to determine whether age alone inversely correlated with TREC levels. Recipients of cord blood grafts had no evidence of decreased TREC induced by immunosuppressive prophylaxis drugs. Compared with unrelated donor grafts, recipients of matched sibling grafts had higher TREC levels. Collectively, these data suggest that thymopoiesis is inhibited by GVHD. Larger studies will be needed to determine the independent contributions of age and preparative regimen to post-transplant thymopoietic capacity.

Functionally inert HIV-specific cytotoxic T lymphocytes do not play a major role in chronically infected adults and children.

The highly sensitive quantitation of virus-specific CD8(+) T cells using major histocompatibility complex-peptide tetramer assays has revealed higher levels of cytotoxic T lymphocytes (CTLs) in acute and chronic virus infections than were recognized previously. However, studies in lymphocytic choriomeningitis virus infection have shown that tetramer assays may include measurement of a substantial number of tetramer-binding cells that are functionally inert. Such phenotypically silent CTLs, which lack cytolytic function and do not produce interferon (IFN)-gamma, have been hypothesized to explain the persistence of virus in the face of a quantitatively large immune response, particularly when CD4 help is impaired. In this study, we examined the role of functionally inert CTLs in chronic HIV infection. Subjects studied included children and adults (n = 42) whose viral loads ranged from <50 to >100,000 RNA copies/ml plasma. Tetramer assays were compared with three functional assays: enzyme-linked immunospot (Elispot), intracellular cytokine staining, and precursor frequency (limiting dilution assay [LDA]) cytotoxicity assays. Strong positive associations were observed between cell numbers derived by the Elispot and the tetramer assay (r = 0.90). An even stronger association between tetramer-derived numbers and intracellular cytokine staining for IFN-gamma was present (r = 0.97). The majority (median 76%) of tetramer-binding cells were consistently detectable via intracellular IFN-gamma cytokine staining. Furthermore, modifications to the LDA, using a low input cell number into each well, enabled LDAs to reach equivalence with the other methods of CTL enumeration. These data together show that functionally inert CTLs do not play a significant role in chronic pediatric or adult HIV infection.

Thymocyte differentiation from lentivirus-marked CD34(+) cells in infant and adult human thymus.

Changes in thymic function and immune system homeostasis associated with HIV infection or chemotherapy have significant effects on the ability of patients to maintain a complete T cell receptor repertoire. Therefore, the development of in vitro systems to evaluate thymic function in children and adults may aid in the understanding of thymopoiesis and the development of new therapies to improve thymic output. Here we use a lentivirus-based gene transfer system to mark CD34(+) cells with EGFP and follow their differentiation into CD4(+) and CD8(+) single positive thymocytes in human thymic organ cultures. Lentivirus-marked cells entered the thymus and were detected in both the cortex and medulla. Pretreatment of the thymus with 2-deoxyguanosine depleted resident thymocytes and significantly increased the percentage of EGFP(+) thymocytes. High frequency gene transfer into CD34(+) cells and maintained expression throughout differentiation allows for the in vitro assessment of thymic function. In thymuses ranging in age from fetal to adult we observed EGFP(+) thymocytes at all stages of development suggesting that thymuses of all ages are capable of accepting new T cell progenitors and contributing to the maintenance of T cell homeostasis.

Putative immunodominant human immunodeficiency virus-specific CD8(+) T-cell responses cannot be predicted by major histocompatibility complex class I haplotype.

Recent studies of human immunodeficiency virus (HIV)-specific CD8(+) T cells have focused on responses to single, usually HLA-A2-restricted epitopes as surrogate measures of the overall response to HIV. However, the assumption that a response to one epitope is representative of the total response is unconfirmed. Here we assess epitope immunodominance and HIV-specific CD8(+) T-cell response complexity using cytokine flow cytometry to examine CD8(+) T-cell responses in 11 HLA-A2(+) HIV(+) individuals. Initial studies demonstrated that only 4 of 11 patients recognized the putative immunodominant HLA-A2-restricted p17 epitope SLYNTVATL, suggesting that the remaining subjects might lack significant HIV-specific CD8(+) T-cell responses. However, five of six SLYNTVATL nonresponders recognized other HIV epitopes, and two of four SLYNTVATL responders had greater responses to HIV peptides restricted by other class I alleles. In several individuals, no HLA-A2-restricted epitopes were recognized, but CD8(+) T-cell responses were detected to epitopes restricted by other HLA class I alleles. These data indicate that an individual's overall CD8(+) T-cell response to HIV is not adequately represented by the response to a single epitope and that individual major histocompatibility complex class I alleles do not predict an immunodominant response restricted by that allele. Accurate quantification of total HIV-specific CD8(+) T-cell responses will require assessment of the response to all possible epitopes.

Assessment of thymic output in adults after haematopoietic stem-cell transplantation and prediction of T-cell reconstitution.

BACKGROUND: The potential benefits of haematopoietic stem-cell transplantation are tempered by the depletion of T-cells accompanying this procedure. We used a new technique which quantifies the excisional DNA products of T-cell-receptor (TCR) gene rearrangement to measure thymic output directly in patients with multiple myeloma, and thus assessed the contribution of the thymus to immune recovery after transplantation. METHODS: We studied 40 patients, 34-66 years of age, who had been randomly assigned myeloablative chemotherapy and autologous peripheral-blood haematopoietic stem-cell transplantation with unmanipulated grafts or grafts enriched for CD34 stem cells. CD4 and CD8 T-cell counts were measured, thymic output was estimated serially until 2 years after transplantation, and percentages of naive T-cells were measured. FINDINGS: The production of substantial numbers of new naive T cells by the thymus could be detected by 100 days post-transplant; there was a significant inverse relation between age and recovery of new T cells. In the CD34-unselected group, numbers of TCR-rearrangement excision circles returned to baseline after 2 years, whereas in the CD34-selected group, numbers at 2 years were significantly higher than both baseline numbers (p=0.004), and 2-year numbers in the unselected group (p=0.046). Increased thymic output correlated with, and was predictive of, increased naive T-cell numbers and broader T-cell-receptor repertoires. INTERPRETATION: Our results provide evidence that the adult thymus contributes more substantially to immune reconstitution after haematopoietic stem-cell transplantation than was previously thought, and therefore could be a target for therapeutic intervention.

Quantification of thymic function by measuring T cell receptor excision circles within peripheral blood and lymphoid tissues in monkeys.

The thymus is the primary organ responsible for the production of mature TCR alpha / beta T cells. Quantification of a DNA excision circle that is produced during TCR rearrangement, termed a signal joint TCR rearrangement excision circle (sjTREC) can be used as a measure of thymic function. Here sjTREC measurement has been applied to two monkey species used as animal models of human disease, rhesus macaques (Asian origin) and sooty mangabeys (African origin). Initial PCR analysis determined that the TCR deltaRec-PsiJalpha rearrangement leading to sjTREC formation occurs in both species. Primers to a DNA sequence conserved in macaques, mangabeys and humans were used in a quantitative competitive PCR assay to quantify sjTREC. We found that as in humans, sjTREC in these two monkey species decline with age. sjTREC are first generated in thymocytes during the early stages of TCR rearrangement. Lymph node CD4(+) and CD8(+) T cells contain more sjTREC than peripheral blood T cell populations, suggesting that recent thymic emigrants home to the lymphoid tissues. The sjTREC level is significantly higher within the peripheral blood CD4(+) and CD8(+) T cells of mangabeys compared to macaques. Removal of the thymus in four macaques led to a profound decrease in peripheral blood sjTREC level by 1 year post-thymectomy, indicating the lack of a significant extra-thymic source of peripheral naive T cells in macaques. Our results indicate that production, trafficking, and proliferation of recent thymic emigrants in these two monkey species represents a useful animal model system for understanding human immunological disorders.

Effect of highly active antiretroviral therapy and thymic transplantation on immunoreconstitution in HIV infection.

The purpose of this study was to determine whether thymic transplantation in addition to highly active antiretroviral therapy (HAART) will restore T cell function in HIV infection. Eight treatment-naive HIV-infected patients with CD4+ T cell counts of 200-500/mm3 were randomized into thymic transplantation and control arms. All patients received HAART (zidovudine, lamivudine, and ritonavir) for 6 weeks prior to transplantation. Thymic transplantation was done without immunosuppression, using postnatal HLA-unmatched cultured allogeneic thymus tissue. Patients were immunized every 6 months with the neoantigen keyhole limpet hemocyanin (KLH) and the recall antigen tetanus toxoid (TT). T cell phenotype and function and T cell receptor rearrangement excision circles (TRECs) were assessed. Thymic allografts were biopsied at 2 months. Six HIV-infected patients completed the study. Four patients received cultured allogeneic postnatal thymic grafts, two others were controls. CD4+ T cell counts increased and T cell-proliferative responses to Candida antigen and TT normalized in all patients. Proliferative responses to KLH developed in three of four transplant recipients and one of two controls. Patients responding to KLH after secondary immunization had greater TREC increases compared with the patients who did not respond. All thymic allografts were rejected within 2 months. In summary, four of six patients developed T cell-proliferative responses to the neoantigen KLH over the first 2 years of HAART. The transplanted thymus tissue, however, was rejected. There was no clear difference in restoration of T cell function in the transplant recipients compared with the controls. Increases in TRECs after initiation of HAART may correlate with improved immune function.