One of the major problems derived from the exposure to ionizing radiation is the impairment of the
immune system. The consequent immune-depression increases the risk of infections and may lead to immunemediated
disorders. The intensity and duration of the immune-compromised phase and its recovery depend on the
dose, dose-rate and quality of radiation. In recent years, there has been a great interest in the effects induced by
protons, both for a better assessment of the health risks in astronauts exposed to solar wind and cosmic radiations
and for a better understanding of their effects in radiotherapy for oncologic patients. In the present study, we
investigated the effects of the in vivo exposure to 2 Gy of integral dose absorbed by medium energy proton beams on
mouse lymphoid spleen cells. The TOP-IMPLART accelerator was used as proton source. Irradiations were performed
in air with pulsed (3.4 s, 10Hz) 27 MeV proton beams. During the exposure, mice were anesthetized in order to keep
them in the right position. Sham-exposed anesthetized age/gender/strain-matched mice were used as controls.
Twenty-four hours and 1 week after irradiation, each mouse was individually analyzed for several parameters
(5 mice/group). Results showed that the number of nucleated cells in the spleen was not significantly affected. Flow
cytometry analyses revealed that the percentages of helper T (CD4), cytotoxic T (CD8) and B (CD19) cells within the
spleen lymphocytes were not altered 24 hours after the exposure. At variance, 1 week after the exposure the frequency
of CD4 (14% vs. 9%) and CD19 (37% vs. 26%) cells reduced. Spleen cells were stimulated with an anti-CD3 antibody
and LPS to induce T cell and B cell activation, respectively. Both T and B cells were functionally impaired by the
exposure. Twenty-four hours after irradiation, T cell proliferation was indeed reduced by 50% in exposed mice
compared with controls. B cells also displayed a reduced cell proliferation in response to the mitogenic stimulus
(-33%). Interestingly, 1 week after irradiation proliferative responses of T and B cells were still compromised. This
first study allowed the conclusion that, in vivo local exposure to protons induced small changes in total spleen cell
number, the frequency of CD4 and B cells being reduced 1 week after the exposure. More interesting, functional
responses, such as T and B cell proliferation were partially compromised. These effects, in spite of the limited area of
exposure, were not recovered after 1 week.