896 
GENETIC AND BIOLOGICAL EFFECTS OF RADIATION 
only repetitive pregraft ECIB at a relatively- 
high transit dose (300-400 rads). Due to in- 
creased susceptibility of goat red blood cells to 
hemolysis by irradiation, relatively little ECIB 
could be given at these dose ranges. Conse- 
quently, a second schedule of repetitive pregraft 
ECIB v^as employed using a low transit dose 
(45-55 rads) but nearly doubling the total 
number of blood volumes irradiated. A third 
schedule included both pre- and post-graft repet- 
itive ECIB using the lov^^ transit dose. The pre- 
liminary results are shovi^n in Table II. These 
findings indicate that: (1) the total number of 
blood volumes irradiated prior to grafting may 
be an important aspect of immunosuppression 
by ECIB; and (2) w^ithout question pre- and 
post-graft ECIB is superior to pre-graft ECIB 
only. This was also true in experiments with 
skin allografts. 
CONCLUSIONS/SUMMARY 
ECIB and ECIL are effective methods for in- 
ducing marked lymphocytopenia and depleting 
the body of an easily mobilizable pool of lympho- 
cytes which circulate from blood to lympho- 
reticular organs and back again. This mass of 
rapidly recirculating cells is composed princi- 
pally of small nondividing lymphocytes which, 
when depleted, is replenished at a very slow 
rate. A fraction of the recirculating pool re- 
moved by extracorporeal irradiation is com- 
posed large lymphocytes. In contrast to small 
lymphocytes, recovery following depletion is 
relatively rapid. 
The sessile pool of lymphocytes in lympho- 
reticular organs is much less affected by extra- 
corporeal irradiation because of the apparent 
slow exchange of cells between blood and tissue 
Table II. — Effect of Extracorporeal Irradiation of 
Blood (ECIB) on the Survival of Renal Allografts 
In Goats. 
ECIB Number Survival 
Schedule of Goats (Days) 
Control _ 8 16.9 
I 8 17.6 
II 9 30.3 
III _ 8 48.1 
Schedule I = Repetitive pregraft ECIB at transit dose of a 330 
rads; Schedule II = Repetitive pregraft ECIB at transit dose of 
a 50 rads; Schedule III = Repetitive pre- and postgraft ECIB at 
transit dose of a 50 rads. 
compartments. Kinetic studies^*' indicate that 
the sessile pool also has a slow replacement rate, 
and from the depletion experiments discussed 
above, it would appear that the total pool size 
is considerably larger than the easily mobiliz- 
able mass of lymphocytes. 
The functional aspects of lymphocytes in the 
mobile versus sessile pools were tested by anti- 
genic stimulation with tetanus toxoid, skin 
allograft and renal allograft rejection. ECIB 
and ECIL only slightly reduced primary and 
secondary responses to tetanus toxoid and mod- 
erately prolonged skin allograft survival. There- 
fore, it would appear that within the sessile pool 
are: (1) antigen-responsive lymphocytes ca- 
pable of initiating primary immune responses, 
(2) sensitized cells capable of eliciting a typical 
secondary immune response, and (3) immuno- 
competent cells able to differentiate into cells 
capable of allograft destruction. Since all of 
these responses were somewhat altered by ECIB 
and ECIL, it can be assumed that the same cell 
types are present in the mobile pool. Therefore, 
immunosuppression to allografts would be most 
effective when extracorporeal irradiation is con- 
ducted before and after grafting. This was par- 
ticularly evident in the prolongation of renal 
allografts by ECIB. 
Further immunosuppression by extracor- 
poreal irradiation could no doubt be achieved by 
"mobilization" of the sessile pool. Several com- 
pounds are known to induce an increase in the 
blood lymphocyte count including heparin ^'^ 
which is used to prevent clotting during ECIB 
or ECIL. Bordetella pertussis and polymeth- 
acrylic acid will increase (2 to 2.5 times) the 
blood lymphocyte count in animals depleted by 
ECIB. The total mass of cells "mobilized" by 
these agents in the depleted state is, however, 
unknown. If the mobilized mass is composed 
essentially of lymphocytes circulating in the 
blood, there is little hope of significantly reduc- 
ing the sessile pool. On the other hand lympho- 
cytes seen in the blood following the injection 
of a lymphocytosis inducing agent may be part 
of a newly established recirculating pool. In this 
case repeated injections followed by ECIB may 
effectively promote lymphocyte depletion and 
immunosuppression. 
ECIB and ECIL continue to be important con- 
