We conducted this research to answer a fundamental question in immunology- Where do antigen inexperienced CD4+ T cells reside?Currently, it is widely accepted that central and effector memory CD4+ T cells originate from naïve T cells after they have encountered their cognate antigen in the setting of appropriate co-stimulation. However, if this were true the diversity of T cell receptor (TCR) sequences within the naïve T cell compartment should be far greater than that of the memory T cell compartment, which is not supported by TCR sequencing data. In fact the vast majority of T cells with a "memory" phenotype do not appear to have undergone a strong clonal expansion event.
In our study we demonstrate that aged mice with far fewer naïve T cells, respond to the model antigen, hen eggwhite lysozyme (HEL), by utilizing the same TCR sequence as their younger counterparts. CD4+ T cell repertoire analysis of highly purified T cell populations from HEL-naive animals revealed that the HEL-specific clones, within these mice, displayed effector and central “memory” cell surface phenotypes even though they had never encountered the HEL antigen. Although cross reactivity to some other foreign antigen exposure could have explained these results, the effector and central “memory” HEL-specific T cells that were detected resided at an extremely low frequencies, identical to their frequency within the naive T cell compartment. Thus, any prior cross recognition event was not strong enough to have induced an expansion of the HEL-specific T cells, at least not one that could be detected above the background level that was seen in the naive T cell compartment. In our experiments the HEL-inexperienced CD4+ T cells were found to reside within the naïve, regulatory, central memory, and effector memory T cell populations at similar frequencies. The vast majority of the other CD4+ T cells within the regulatory and memory populations were also unexpanded.
These findings support a new paradigm for CD4+ T cell maturation in which a specific clone can undergo a differentiation process to exhibit a “memory” or regulatory phenotype without having undergone a clonal expansion event. They also demonstrate that a foreign-specific T cell is just as likely to reside within the regulatory T cell compartment as it would the naïve compartment, arguing against the specificity of the regulatory T cell compartment being skewed towards self-reactive T cell clones. Finally, we demonstrate that the same set of foreign T cell clones are repetitively generated throughout adulthood.