T cell responses are initiated by antigen and promoted by a range of costimulatory signals. Understanding how T cells integrate alternative signal combinations and make decisions affecting immune response strength or tolerance poses a considerable theoretical challenge. Here, we report that T cell receptor (TCR) and costimulatory signals imprint an early, cell-intrinsic, division fate, whereby cells effectively count through generations before returning automatically to a quiescent state. This autonomous program can be extended by cytokines. Signals from the TCR, costimulatory receptors, and cytokines add together using a linear division calculus, allowing the strength of a T cell response to be predicted from the sum of the underlying signal components. These data resolve a long-standing costimulation paradox and provide a quantitative paradigm for therapeutically manipulating immune response strength.

There are two major mechanisms reported to prevent the autoreactivity of islet-specific CD8 ؉ T cells: ignorance and tolerance. When ignorance is operative, naïve autoreactive CD8 ؉ T cells ignore islet antigens and recirculate without causing damage, unless activated by an external stimulus. In the case of tolerance, CD8 ؉ T cells are deleted. Which factor(s) contributes to each particular outcome was previously unknown. Here, we demonstrate that the concentration of self antigen determines which mechanism operates. When ovalbumin (OVA) was expressed at a relatively low concentration in the pancreatic islets of transgenic mice, there was no detectable cross-presentation, and the CD8 ؉ T cell compartment remained ignorant of OVA. In mice expressing higher doses of OVA, cross-presentation was detectable and led to peripheral deletion of OVA-specific CD8 ؉ T cells. When crosspresentation was prevented by reconstituting the bone marrow compartment with cells incapable of presenting OVA, deletional tolerance was converted to ignorance. Thus, the immune system uses two strategies to avoid CD8 ؉ T cell-mediated autoimmunity: for high dose antigens, it deletes autoreactive T cells, whereas for lower dose antigens, it relies on ignorance.antigen presentation ͉ transgenic mice ͉ ovalbumin ͉ apoptosis ͉ autoimmunity

To better understand the antigenic requirements for cross-presentation, we compared the in vivo efficiency of presentation of cell-associated vs soluble OVA with the OT-I (CD8) and OT-II (CD4) TCR transgenic lines. Cross-presentation of cell-associated OVA was very efficient, requiring as little as 21 ng of OVA to activate OT-II cells and 100-fold less to activate OT-I cells. In contrast, soluble OVA was presented inefficiently, requiring at least 10,000 ng OVA for activation of either T cell subset. Thus, cell-associated OVA was presented 500-fold more efficiently than soluble OVA to CD4 T cells and 50,000-fold more efficiently to CD8 T cells. These data, which represent the first quantitative in vivo analysis of cross-presentation, show that cell-associated OVA is very efficiently presented via the class I pathway.