See Nov. 9 Web notes for discussion of von Boehmer's findings. His data was illustrative of a number of points. In his system he saw a complete absence of DP thymocytes, leading most people to believe that the DP thymocyte was the target of a clonal deletion process. I think this is 'dangerous' and should be discouraged. Negative selection best refers to the processes by which autoreactive specificities are removed or disabled from a repertoire.
Clonal deletion is one mechanism by which negative selection can be achieved.
Other mechanisms include clonal arrest, receptor editing, anergy, etc. Positive selection Von Boehmer's data also revealed something else. When he introduced the transgene into female mice of the H-2 b haplotype, he found a preponderance of CD8 SP cells. Remember that Z and D found that T cells isolated from mice of one strain could only recognize antigen if it was presented by APC of the same strain. Other data indicated that this learning process occurred in the thymus.