EFFECTS ON IMMUNE RESPONSE 107Figure 5.9. Effect of irradiation upon anti-SRBC response. C57BL/6 spleen cells from
female donors were exposed to indicated doses of radiation in vitro and
incubated in 1-mL cultures with SRBCs as antigen. The number of anti-SRBC
plaque-forming cells (PFCs) was determined on day 4. The results are
expressed as a percentage of the control (0 rad) response. The numbers in
parentheses indicate the number of experiments included in the calculation
of the indicated value. The actual data from a single experiment (dotted line)
are included for comparative purposes. Adapted from Anderson and
Lefkovits.16
Although the intrathymic deletion of T cells probably is genetically initi
ated, perhaps by permitting an influx of calcium, radiation-induced inter
phase death is clearly triggered by the physical event per se. The end result,
interphase death, may be due to either a direct or an indirect effect, such as
free radical formation. Whatever the initial event, however, it is easy to
envision a series of steps that culminate in endonuclease activation.
As noted previously, low-dose augmentation of the immune response
appears to relate to the functional loss of an immunoregulatory cell of the T
cell lineage. The requisite temporal relationships among irradiation, intro
duction of antigen (or mitogen), and an augmented response suggest that
this phenomenon may be due to interphase cell death similar, at least mor
phologically, to that observed among normal T cells during their differenti
ation within the thymus. Although spontaneous and radiation-initiated
apoptosis have been particularly well studied in the thymus, it must be
reemphasized that the phenomenon is not limited to T cells. In fact, a recent
report documents the same phenomenon in B cells.27
Is there an evolutionary basis for the extreme radiosensitivity of some
types of lymphocytes? Does it offer some type of competitive advantage to
the host? Several hypothetical arguments can be advanced:
- Coincidence. It is also possible that the relationship between apoptosis and
radiation-induced interphase death is purely coincidental, that radiation