We investigated CD4+ and CD8+ T cell turnover in both healthy and HIV-1–infected adults by measuring the nuclear antigen Ki-67 specific for cell proliferation. The mean growth fraction, corresponding to the expression of Ki-67, was 1.1% for CD4+ T cells and 1.0% in CD8+ T cells in healthy adults, and 6.5 and 4.3% in HIV-1–infected individuals, respectively. Analysis of CD45RA+ and CD45RO+ T cell subsets revealed a selective expansion of the CD8+ CD45RO+ subset in HIV-1–positive individuals. On the basis of the growth fraction, we derived the potential doubling time and the daily turnover of CD4+ and CD8+ T cells. In HIV-1–infected individuals, the mean potential doubling time of T cells was five times shorter than that of healthy adults. The mean daily turnover of CD4+ and CD8+ T cells in HIV-1–infected individuals was increased 2- and 6-fold, respectively, with more than 40-fold interindividual variation. In patients with <200 CD4+ counts, CD4+ turnover dropped markedly, whereas CD8+ turnover remained elevated. The large variations in CD4+ T cell turnover might be relevant to individual differences in disease progression.

HIV-1 viremia is a marker of choice for staging, prognosis, and monitoring treatment efficiency in HIV infection. Among the commercial assays, the Amplicor HIV Monitor (Roche, Basel, Switzerland) test has the highest sensitivity for HIV-1 RNA quantitation in plasma with a detection limit of 200 copies per milliliter. To measure HIV-1 viremia below this threshold, boosted versions of the Amplicor assay were developed by adding a centrifugation step prior to RNA extraction and by decreasing dilution factors. In the boosted version, the increase in analytical sensitivity for HIV-1 RNA detection directly correlates with the input of plasma. For 1,500 microl of plasma, the sensitivity of the assay increases by a factor of 30. For routine clinical analysis, we use a boosted assay format with an input plasma volume of 500 microl and a lower detection limit of 20 copies/milliliter. Coefficients of intra- and interassay variation are similar to those reported for the standard assay (approximately 30%). Thirteen (45%) of 29 plasma samples of HIV-infected individuals with undetectable viremia in the standard assay had detectable viremia between 20 and 200 copies/milliliter.

It has been proposed that antibodies can mimic the binding of a receptor to its ligand and that anti-idiotype antibodies raised against such antibodies can be used to identify the receptor. A large number of antibodies have been raised against CD4, the receptor on T cells for the envelope glycoprotein gp120 of the human immunodeficiency virus, and the site at which gp120 binds to CD4 has been delineated. It has therefore become possible to contrast the fine specificities of a natural ligand (gp120) and antibodies that interact with the receptor at the same site. Here we report that out of a panel of 225 anti-CD4 antibodies, only one showed fine binding specificity that was broadly like that of gp120, but the evidence was against this being an exact mimic. Thus the data indicate that the production of antibody mimics will occur very rarely or not at all and that the anti-idiotype approach is unlikely to be useful. This contention is supported by a review of the results of attempts to use this approach. Taking strict criteria for success, there is no example for which the anti-idiotype approach has led to the discovery of a previously undescribed receptor or other protein of interest.

Slllnnlal'y CD4 is the primary receptor for the human irnmunodeficiency virus type 1 (HIV-1). Early mutational studies implicated a number of residues of CD4, centered in the region 41-59, in binding to gpl20. However, further mutational analyses, together with studies using inhibitory antibodies or CD4-derived peptides, have suggested that other regions of CD4 are also involved in binding or postbinding events during infection. To resolve these ambiguities, we used rat CD4 mutants in which particular regions were replaced with the corresponding sequence of human CD4. We have previously shown that some of these are able to bind HIV-1 gpl20, and here we test their ability to act as functional receptors. We find that the presence of human CD4 residues 33-62 is enough to confer efficient receptor function to rat CD4, and we conclude that it is unlikely that regions of CD4 outside this sequence are involved in specific interactions with HIV-1 during either infection or syncytium formation.T he CD4 cell surface glycoprotein is the primary receptor for HIV-1, and its high affinity interaction with the viral glycoprotein gp120 is the initial step in HIV-1 infection of most susceptible cells (1, 2). The extracellular region of CD4 contains four Ig superfamily domains, the amino terminal of which closely resembles an Ig light chain variable domain (3, 4). The three-dimensional structure of the amino-terminal two domains has been confirmed by x-ray crystallography (5, 6). In vitro mutational analyses have identified residues 41-59 within the C'-C" ridge of CD4 domain 1 as the probable binding site for gpl20 (5, 7-11).The postbinding events that lead to fusion of the virus and cellular membranes are unclear. CD4 may induce conformational changes in gpl20 that expose the fusogenic region of gp41 (12-15). It has been proposed that regions of CD4 outside the gp120-binding site might be involved in this process (16-18). For example, mAbs that bind the F-G turn (equivalent to the CDR3 loop of Igs) block syncytium formation and viral entry while only poorly inhibiting the binding of gpl20 or HIV-1 (16). Derivatized peptides corresponding to residues 81-92 of human (h) 1 CD4, which encompass this loop, inhibit postbinding events (17). Another study implicated this region by demonstrating that chimpanzee 1 Abbreviations used in this paper: h, human; m, mutant; m.o.i., multiplicity of infection; r, rat.CD4 and hCD4 with a chimpanzee CDR3 loop were unable to mediate HIV-l-dependent syncytium formation, unlike wild-type hCD4 or chimpanzee CD4 with a hCDR3 loop (19).Rat (r)CD4 shares only 50% homology with hCD4 and does not function as a receptor for HIV-1. We have taken advantage of this to determine the regions of CD4 involved in HIV-1 infection by replacing rCD4 sequence with the equivalent hCD4 residues. Using this approach initial studies established that the gp120 binding site of CD4 is encompassed by residues 33-62 of domain 1 (20). In the present study we have taken the analysis a stage further and determined whether regions o...