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Diversity of Human αβ T Cell Receptors

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Science  19 May 2000:
Vol. 288, Issue 5469, pp. 1135
DOI: 10.1126/science.288.5469.1135a

Arstila et al. (1) estimated an average diversity of 9 × 105 different β chains and 4.5 × 105 different α chains in the human naı̈ve T cell repertoire. To calculate the total T cell repertoire diversity, the β-chain diversity was estimated within a certain variable (V) gene family, Vα12+, comprising 2.5% of the total α-chain repertoire. Finding in this particular family an estimated total of 6 × 105 different β chains (i.e., two-thirds of the total β-chain repertoire), Arstila et al. suggested that the total T cell receptor (TCR) diversity comprises at least (6 × 105) × 40 = 2.4 × 107 different αβ combinations (1). The authors acknowledge that this is only a lower bound, because the calculation assumes that the β chains that do bind at least one Vα12 chain bind only one of the 4.5 × 105 different α chains in the Vα12+ family. If each β chain found within the Vα12+ family were to bind an average of n different Vα12 chains instead, the total estimated TCR diversity would be n-fold higher than this lower bound.

Arstila et al. estimated an upper bound of 108different αβ combinations (1). Pre–T cells having rearranged a β chain expand 1000-fold before the α chain is rearranged, and only 10% of these cells leave the thymus to enter the mature repertoire. Thus, it was argued that each β chain can maximally pair with any of about 100 different α chains.

This is indeed correct for all descendants of any particular pre–T cell having rearranged a particular β chain—but another pre–T cell rearranging the same β chain may bind to 100 different α chains. Thus, to calculate the upper bound on TCR diversity, one has to consider the frequency with which identical β-chain rearrangements are expected. This frequency can be estimated from the turnover rate of the naı̈ve T cell repertoire. In human adults, the total body production of naı̈ve T cells has been estimated at about 108 per day (2), a figure obtained from recovery rates following T cell depletion (2) and from an estimated 0.1% turnover (3) in a pool of 1011naı̈ve T lymphocytes. Assuming that most of this production is of thymic origin (4) and that more than 90% of the cells die before leaving the thymus (1), this implies a daily production of at least 109 pre–T cells. The 1000-fold expansion of the pre–T cells (1) before α-chain rearrangement implies that approximately 106 β chains should be made every day. Because this is close to the Arstila et al. estimate of total β-chain diversity, every β chain should be rearranged about every day.

Over the 1000-day expected life-span (2, 3) of the progeny of a pre–T cell expressing a single β chain, therefore, 1000 recurrences of the same β-chain rearrangement might be expected. Hence, the upper bound for the total TCR diversity could easily be 1000-fold larger than calculated by Arstila et al. Such an upper bound, at 1011, would allow almost every T cell in the naı̈ve repertoire to have a unique TCR. The true TCR diversity may be several fold lower, however, owing to factors such as proliferation after the α-chain rearrangement and possible restrictions in αβ-chain pairing.

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Diversity of Human αβ T Cell Receptors

Response: Keşmir et al. argue that although any developing TCR β chain will be paired at most with 100 different α chains, the same β chain may appear repeatedly and garner other sets of 100 α chains, increasing the total αβ TCR diversity from the 108 we estimated (1). We studied the diversity of the human αβ TCR in the blood of healthy adult donors at a given moment, not over time. Also, we did not measure the upper limit of α-to-β pairing; our estimate was based on what is known of αβ T cell development and TCR rearrangement. Thus, the comment of Keşmir et al. actually goes beyond our data.

Because any expansion after α-chain rearrangement will increase only clone size, not diversity, the argument of Keşmir et al. hinges on the assumption that the estimated total turnover of naı̈ve T cells equals thymic production of pre–T cells. That assumption is incorrect, however, and ignores the well-documented role of post-developmental division in the maintenance of the naı̈ve T cell population, especially in adults. Murine T cells may go through up to six cell cycles after α-chain rearrangement even before emigrating from the thymus (2). Haynes et al., cited by Keşmir et al., specifically argued for “minimal contributions of the thymus to maintenance or reconstitution of the peripheral pool of T cells . . .” in humans [(3), p. 457], and showed that the presence or absence of thymic function and even the surgical removal of the thymus had no impact on the reconstitution of the T cell compartment, including the naı̈ve CD4+ cells, in treated HIV-infected individuals. Naı̈ve T cells, long after having completed TCR rearrangement, clearly have a considerable capacity for self-renewal.

The suggestion of Keşmir et al. can also be viewed as a question of clone size. If the size of the repertoire is 108 different TCRs, as we suggest, the average clone among 1011 naı̈ve T cells would consist of 1000 cells, the progeny of a single intrathymic α-chain rearrangement after 10 cell cycles. These cycles should therefore be detectable in the naı̈ve T cell population, and indeed this appears to be the case. Studying the disappearance of cells damaged by therapeutic irradiation, McLean and Michie (4) concluded that, on average, naı̈ve T cells divide once every 3.5 years and die after 20 years, which suggests six post-thymic cell cycles in the life-span of an average naı̈ve T cell. Other experimental approaches have suggested higher division rates. From age 25 to 70 years the mean telomere length in the naı̈ve T cell population decreases from 9.5 kb to 8.0 kb, so an estimated loss of 50 to 100 base pairs (bp) per cell cycle translates to 7 to 13 divisions during the 20-year life-span of naı̈ve cells (5). De Boer and Noest have argued that this estimate of telomere loss is too high; their estimate, 35 to 70 bp per cycle (6), would mean 10 to 19 cycles. At any given time the fraction of naı̈ve T cells in cell cycle is 0.8% (7), which suggests a rate as high as one division per 125 days, or 60 cycles per life-span. The available data thus can easily accommodate 10 divisions producing the average naı̈ve clone.

Studies on the frequency of antigen-specific T cell precursors provide an independent line of evidence that points to a diversity close to what we proposed. A conservative estimate of the frequency of such precursors in the naı̈ve repertoire would be one per million; some studies have reported significantly higher frequencies (8, 9). Thus, a total repertoire of 108 TCRs would predict an epitope-specific response to consist of 100 clones, while Keşmir et al.'s repertoire of 1011 TCRs predicts a composition of 100,000 responding clones. The existing literature is more compatible with our prediction (10–14). Thus, we submit that the phenomenon that Keşmir et al. postulate, although in principle possible, has little impact on the total diversity.

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