The aged microenvironment impairs BCL6 and CD40L induction in CD4⁺ T follicular helper cell differentiation

Weakened germinal center responses by the aged immune system result in diminished immunity against pathogens and reduced efficacy of vaccines. Prolonged contacts between activated B cells and CD4⁺ T cells are crucial to germinal center formation and T follicular helper cell (Tfh) differentiation, but it is unclear how aging impacts the quality of this interaction. Peptide immunization confirmed that aged mice have decreased expansion of antigen-specific germinal center B cells and reduced antibody titers. Furthermore, aging was associated with accumulated Tfh cells, even in naïve mice. Despite increased numbers, aged Tfh had reduced expression of master transcription factor BCL6 and increased expression of the ectonucleotidase CD39. In vitro activation revealed that proliferative capacity was maintained in aged CD4⁺ T cells, but not the costimulatory molecule CD40L. When activated in vitro by aged antigen-presenting cells, young CD4⁺ naïve T cells generated reduced numbers of activated cells with upregulated CD40L. To determine the contribution of cell-extrinsic influences on antigen-specific Tfh induction, young, antigen-specific B and CD4⁺ T cells were adoptively transferred into aged hosts prior to peptide immunization. Transferred cells had reduced expansion and differentiation into germinal center B cell and Tfh and reduced antigen-specific antibody titers when compared to young hosts. Young CD4⁺ T cells transferred aged hosts differentiated into Tfh cells with reduced PD-1 and BCL6 expression, and increased CD39 expression, though they maintained their mitochondrial capacity. These results highlight the role of the lymphoid microenvironment in modulating CD4⁺ T cell differentiation, which contributes to impaired establishment and maintenance of germinal centers.