The adaptive immune system uses several strategies to generate a repertoire of T-and B-cell antigen receptors with sufficient diversity to recognize the universe of potential pathogens. In ␣␤ T cells, which primarily recognize peptide antigens presented by major histocompatibility complex molecules, most of this receptor diversity is contained within the third complementarity-determining region (CDR3) of the T-cell receptor (TCR) ␣ and ␤ chains. Although it has been estimated that the adaptive immune system can generate up to 10 16 distinct ␣␤ pairs, direct assessment of TCR CDR3 diversity has not proved amenable to standard capillary electrophoresis-based DNA sequencing. We developed a novel experimental and computational approach to measure TCR CDR3 diversity based on single-molecule DNA sequencing, and used this approach to determine the CDR3 sequence in millions of rearranged TCR␤ genes from T cells of 2 adults. We find that total TCR␤ receptor diversity is at least 4-fold higher than previous estimates, and the diversity in the subset of CD45RO ؉ antigen-experienced ␣␤ T cells is at least 10-fold higher than previous estimates. These methods should prove valuable for assessment of ␣␤ T-cell repertoire diversity after hematopoietic cell transplantation, in states of congenital or acquired immunodeficiency, and during normal aging. (Blood. 2009;114:4099-4107)

Diversity in T-lymphocyte antigen receptors is generated by somatic rearrangement of T-cell receptor (TCR) genes and is concentrated within the third complementarity-determining region (CDR3) of each chain of the TCR heterodimer. We sequenced the CDR3 regions from millions of rearranged TCR β chain genes in naïve and memory CD8+ T-cells of seven adults. The CDR3 sequence repertoire realized in each individual is strongly biased toward specific Vβ-Jβ pair utilization, dominated by sequences containing few inserted nucleotides, and drawn from an effective sequence space 250-fold smaller than predicted. Surprisingly, the overlap in the naïve CD8+ TCRβ CDR3 sequence repertoires of any two of the individuals is ~1000-fold larger than predicted and essentially independent of the degree of HLA matching.

Normalised values of partial ionisation cross sections for the rare gases (He+, Ne+-Ne3+, Ar+-Ar3+, Kr+-Kr4+ and Xef-Xe5+) have been measured from threshold to 1000 eV using a pulsed electron beam and ion extraction technique. The cross sections have been obtained for the singly ionised species by normalising the relative data with the results of Rapp and Englander-Golden below the onset of production of the doubly ionised species. These cross sections for the singly ionised species have then been used to calibrate the mass transmission efficiency of the ion extraction + analyser+ detection system by the relative flow technique. The mass transmission curve thus obtained has been employed to determine the absolute cross sections of the multiply ionised species. Summation of the individual partial cross sections with proper weighting for charge is employed to obtain the total ion cross sections. The measured partial cross sections are fitted to an empirical formula for the ease of future use.

A new method for measuring absolute electron-impact differential cross sections (DCS) for elastic scattering in gases is described. The technique uses a well-defined crossed-beam scattering geometry in which the relative target densities of an unknown and a secondary standard gas are accurately determined by a calibrated flowmeter and a pressure gauge. The method is applied to the measurement of elastic scattering from H2 (which, in the present work, also includes rotational excitation) relative to elastic scattering from He, the secondary standard. The relative DCS for H2 are then placed on the absolute scale by using recently measured absolute elastic DCS for He. Measurements for H2 are reported at incident electron energies of 3–75 eV, and in the angular range 20°–135°. Differential and integral cross sections at each energy are compared with previous measurements and recent theoretical calculations.

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Cross sections for the production of singly charged ions by electron impact on N2O and NO have been obtained by utilizing the relative flow technique in which the accurately known values of cross sections for the production of singly charged ions of rare gases have been used for normalization. By summing the cross sections for direct ionization and dissociative ionization, the total ionization cross sections have been obtained and compared with the previously available data. Energies for the appearance of various ions have also been measured and compared with previous determinations from photoelectron spectroscopy.