CD44 and atrial fibrillation: Although the absolute number of CD4+ T cells in PPs was decreased in GF and GF-AF mice compared with SPF mice, with no significant difference between GF and GF-AF mice (Figure 3D), the frequency of CD4+ T cells with naïve phenotype (CD62L+CD44−) (25) among CD4+ T cells was higher in GF-AF mice than in GF mice (Figure 3E).