Lymphocyte lifespan, immunological memory and retroviral infections

Monitoring hprt mutant frequency over time in T-lymphocytes of people accidentally exposed to high doses of ionizing radiation

Modern technologies have provided the opportunity to monitor mutations in people in vivo. The subjects of this study were accidentally exposed to 137Cesium in a radiological accident that occurred in September 1987 in Goiânia, Brazil, during which more than 150 people received doses greater than 0.1 Gy and as high as 7 Gy. The objective of this study was to determine how long the hprt mutant T-cells in the peripheral blood contribute to mutant frequency by examining the time-course of the T-lymphocyte response to ionizing radiation. This report describes the results obtained over a period of 2.3 to 4.5 years subsequent to the accident, from 11 subjects with doses ranging from 1 to 7 Gy, and from nine control subjects selected from the same population. The mean In MF (+/- SE) of the control group was 2.5 (+/- 0.2) + In10(-6). The exposed group had a significantly increased mutant frequency; the mean In MF (+/- SE) were 3.3 (+/- 0.3) + In10(-6), 2.8 (+/- 0.2) + In10(-6), and 2.3 (+/- 0.2) + In10(-6), in the years 1990-1992 respectively. Based on the decline of mutant frequency and using Buckton's models [Buckton et al. (1967): Nature 214:470-473], we demonstrated that mutant T-cells have a short-term memory with a half-life of 2.1 years. This relatively short half-life limits the effective use of the hprt assay as the method of choice to monitor past exposure. The data also demonstrate a positive correlation with age, and an inverse correlation with plating efficiency.

Suggestions concerning the relationship between mutant frequency and mutation rate at the hprt locus in human peripheral T-lymphocytes

Mutant frequency is defined as the proportion of mutant cells in a population and is readily estimated. It should be distinguished from mutation rate, which relates to the rate at which mutation events arise, and is generally expressed as events per cell division. Since one mutation event may give rise to one or many mutant cells, depending on the generation in which it has arisen, the relationship of mutant frequency to the underlying mutation rate is complex. A large number of estimates of mutant frequency at the hprt locus in human lymphocytes are available, from our two laboratories among others. From our two extensive data sets, we have determined median hprt mutant frequencies of different age groups and used the method of Lea and Coulson (J. Genet., 49, 1949, 264-285) to attempt to estimate the underlying mutation rate at this locus. It is in principle possible to obtain estimates of mutation rate from the mutant frequency in newborns, from the increase in mutant frequency with age, and from the difference between the upper and lower quartile mutant frequencies. We discuss reasons for the discrepancies between these estimates and argue that the best estimate can probably be obtained from the increase in mutant frequency with age. We arrive at an estimate of mutation rate to 6-thioguanine resistance at the hprt locus of about 5 x 10(-7) mutation events per nominal cell division.

AIDS as immune system activation. Key questions that remain

Immune system activation is gaining attention as a central part of HIV pathogenesis. Although there is no consensus yet as to the source of the signal or the result of the signalling, this line of thinking represents a significant shift in the paradigm away from considering HIV disease like any other cytopathic viral infection. Hopefully, completion of studies focussed on this approach will lead to more complete understanding of AIDS and more effective therapies, and will at least bring to the fore some of the central unanswered questions in modern cellular immunology.