IMMUNOLOGICAL COMPETENCE OF SMALL LYMPHOCYTES 237 



In nearly all investigations, heterogeneous populations of 

 leucocytes have been transplanted. It has thus been shown that 

 immunologically competent cells are present in the spleen, 

 lymph nodes, bone marrow, thymus, skin, blood — indeed, 

 probably in all adult tissues where leucocytes are prevalent. 

 Following Simonsen's (1957) demonstration that adult cells 

 capable of inciting transplantation disease propagated in chick 

 embryos, Terasaki (1959) attempted to identify the cell type 

 responsible. This study revealed that blood lymphocytes from 

 adults regularly produce graft-versus-host reactions in chick 

 embryos, whereas monocytes have no detectable effect. Taken 

 together, the fmdings of Simonsen and of Terasaki impHcated the 

 lymphocytic system as the source of the mischief-making cells in 

 birds. With this background and the knowledge that about 

 80 per cent of the leucocytes in mouse peripheral blood are 

 lymphocytes (Dunn, 1954), we proceeded to develop a method 

 for obtaining such lymphocytes in pure preparations for testing 

 in the graft-versus-host assay. Our efforts received additional 

 impetus when Anderson, Delorme and Woodruff (i960) and 

 Billingham and co-workers (i960) discovered that injection of 

 homologous thoracic duct lymphocytes regularly causes runt 

 disease in newborn rats. Interest was then centred on whether large 

 and medium lymphocytes capable of division or small lympho- 

 cytes presumed to be incapable of mitosis were effective. We were 

 also intrigued by the challenge that no defmitive function had 

 ever been ascribed to small lymphocytes (cf Perry et ah, 1959). 



The objectives of the present study were (i) to obtain pure 

 preparations of mouse lymphocytes and assess the competence of 

 these cells in the adult graft-versus-neonatal host system; (2) to 

 evaluate the transplantation reactions produced in relation to such 

 factors as cell dosage, age of recipients at time of injection, patho- 

 logy and median survival time; (3) to ascertain whether these 

 lymphocyte preparations contained cells capable of further 

 rephcation and prolonged survival in neonatal hosts. 



