Antigen dispersal is a key determinant of efficient cross-priming

Cross-priming is a critical component of T cell responses to cancers and viruses, and involves transfer of antigen from antigen donor cells to the antigen presenting cells. In spite of the centrality of antigen in this process, the influence of the quantity of antigen expressed by the antigen donor cell on the efficiency of cross-priming remains unexamined. Here, I describe the creation of a novel system where the model antigen ovalbumin is expressed in P815 (d haplotype) cells under the control of an inducible promoter, producing a large amount of antigen synthesis upon induction. However, even in the un-induced condition, a very low level of ovalbumin can be detected using sensitive methods to amplify the weak signal. I have used titrated quantities of uninduced and induced cells, expressing vastly different quantities of ovalbumin, and have monitored cross-priming of the endogenous anti-OVA CD8+ T cell response quantitatively in C57BL/6 mice (b haplotype), using in vivo cytolytic T lymphocyte assays. The experiments show, paradoxically, that minute quantities of antigen expressed by a large number of cells are far more efficient at cross-priming than exponentially larger quantities of antigen expressed by fewer cells. This result is not due to the effects of non-specific cellular adjuvants; rather, my data suggests it is linked to highly efficient transfer of antigen to cross-priming antigen presenting cells. I have further defined a parameter of this efficiency as the dispersal of low-level antigen into a larger number of delivery vehicles, which itself enhances antigen availability for cross priming by an order of magnitude in a minimalist in vitro system. Thus, I conclude that antigen transfer in cross priming is in some ways a quantized process, wherein the number of encounters between donor and acceptor cells, or the number of opportunities for antigen transfer, is as important to cross-priming as the quantity of antigen expressed in the antigen donor cell and available for sampling by professional antigen presenting cells.