Compared to the T cell compartment, i.e., [41, 45], the effects of ageing on professional APC such as B cells, monocytes/macrophages, and dendritic cells (DC) have received much less attention. There is evidence that monocytes and macrophages from aged mice have a reduced functional potential [46, 47]. Very little is known about DC changes with age, although their number in the epidermis decreases with age [48, 49]. Studies of age-related effects on B cell-mediated immunity are also not as advanced as those of the T cell immune response .
Similar to , we observed that the expression of MHC antigens on splenic lymphocytes in C57BL/6 mice increases with age. Our data indicated that C57BL/6 animals have higher proportion of B cells at all ages than BALB/c mice. Similarly, previous reports suggests that the relative proportion of B220+ cells is high in C57BL/6 and intermediate in BALB/c mouse strains [8, 51]. We also found that the % of CD19+ B cells increases with age in peripheral blood of both strains, while the increases in splenic B cells are prominent in C57BL/6 strain only. This finding, at least for PBMC, differs from earlier reports suggesting that the number of peripheral blood B lymphocytes (B220+) in C57BL/6 and BALB/c mice does not change with age [8, 52]. This difference may be due to the different markers used to identify B cells (CD19 versus B220) or to the different sex of the animals (all males used in our study; females in ; males and females in ).
Our comparison of the kinetics of MHC class II, CD19, and the adhesion molecules CD11b and CD11c expressed on professional APC indicated that, except at the age of one month, C57BL/6 mice had higher % of CD19+ cells. At the same time, we did not observe any strain-related differences in the proportion of CD11b+ or CD11c+ APC in the same age groups. Our data that peripheral blood T and B cells virtually do not express CD11b or/and CD11c are in agreement with the report that the population of peripheral blood CD11b+/CD11c+ APC are predominantly DC . The proportion of CD11b+/CD11c+ APC was significantly greater in 1, 3, and 10 months old C57BL/6 mice than in the BALB/c mice. As previously reported, in general, the proportion of CD11b+/CD11c+ DC in peripheral blood is relatively small . Therefore, increases in the % of DC could not contribute dramatically to the increases in the % of cells expressing MHC class II molecules. Based on the kinetics of MHC class II, CD19, CD11b and CD11c markers in aged animals, we suggest that strain-related differences in MHC class II+ cells were most likely due to the differences in the number of CD11b+/CD11c+ DC in 1 month old mice, CD19+ B cells and DC at the age of 3 months, and CD19+ B cells in 5- and 18-month-old animals.
Unlike in PBMC populations, the spleen cell expression of MHC class II, but not of CD19, was significantly higher in 1-month old C57BL/6 mice. At the same time we did not find any strain-related differences in the % of CD11b+, CD11c+ or CD11b+/CD11c+ APC at this age. Most likely, the CD11b-/CD11c- DC populations contribute to the strain differences at this age. Our data suggest that 18-month-old mice from both strains did not differ in the % of CD19+ B cells. Interestingly, we found significant strain differences in the % of CD11b+, CD11c+ and CD11b+/CD11c+ APC in 18 months old animals. This suggests that the strain difference in the % of MHC class II + APC at the age of 18 months was due to the differences in the % of monocytes and DC [53, 54].
In contrast to previously reported data that T cell number increases dramatically with age in mice bred by a cross between CB6F1 mothers and C3D2F1 fathers , we conclude that age does not affect the % of total T cells in mouse PBMC from the strains used here. However, differences were evident in splenic T cell populations. C57BL/6 mice had a greater CD3+ T cell % than the BALB/c strain up to the age of 10 months, followed by a substantial decline up to the age of 18 months, resulting in significantly lower % of T cells than their BALB/c counterparts. These results might be explained by the age-related differential accumulation of T cell clones in the spleens of mice from different strains.
Our data are in accord with earlier reports that the prototypical Th2-type strain BALB/c has a greater % of CD4-bearing cells. We observed that at all age groups BALB/c mice have higher % of CD4+ cells than C57BL/6 strain in both PBMC and spleen [8, 51]. Similarly, our finding that the % of CD4+ cells decreases with age in peripheral blood regardless of strain agrees with previous reports [17, 52, 55].
The decline of CD4+ cells with age that occurs in circulation was not observed in the spleens of BALB/c mice. Similar, other groups have found little no or change with age in CD4+ cell proportions in spleen of several inbred mouse strains, including BALB/c [50, 56–58]. Yet, in other mouse strains, including C57BL/6, the splenic CD4+ T cells decreased with age [17, 52, 59], which is what we observed here, albeit of smaller magnitude than in the blood. Thus, CD4+ T cell clonal expansion, which has been described in previous reports apparently occurs with age only in the spleens of the prototypical Th2-type strain BALB/c [17, 18].
Peripheral blood CD8+ T cells fluctuated with age in both strains. Fluctuations were greater overall in the C57BL/6 strain, as has been previously observed [8, 51]. Our finding that the % of CD8-bearing lymphocytes in peripheral blood does not decrease at the age of 18 months is in accord with previous reports [17, 52, 18, 55]. However, others have reported significant increases in the number of CD8+ T cells in circulation [2, 58] or moderate declines in the number of cytotoxic T cells .
In general, our results support a gradual, age-dependent shift from naïve CD44neg/low cells towards an increase in CD44med/high cell populations, representing activated or memory phenotypes in mice and humans [8, 28, 52, 55, 60, 61]. The data presented here are significant regarding these earlier observations in two major ways. First, we found strain differences in the distribution of naïve and activated and/or memory cells in splenic CD4+ and CD8+ T cell populations; BALB/c mice had more activated/memory T-cells and less naïve T cells than C57BL/6 mice. Second, the overall kinetics of CD4+ naïve and activated/memory phenotypes were similar between the two strains and resembled the kinetics of their CD8+ naïve and activated/memory T cells.
Overall, the strain-related phenotypic changes in the splenic APC and T cell populations did not always correlate with the changes in the APC and T cells residing in peripheral blood. There were prominent strain differences in both PBMC and spleen populations in the % of CD4+ (higher in the BALB/c strain), CD8+ T cells (higher in C57BL/6 mice) and CD11b+/CD11c+ APC (higher in C57BL/6 mice). Other strain differences however, were present only in PBMC. Namely, the differences in the % of MHC class II + and CD19+, which were greater overall in the C57BL/6 strain than in BALB/c mice. Of note, the strain difference in the % of CD3+ T cells was only evident in the spleens but not in the peripheral blood. The C57BL/6 strain had greater % of T cells than BALB/c mice. Because of the differences in the composition of the peripheral blood cells and splenocytes  spleen cells could not always be used as surrogates for peripheral blood .
Age as a factor, influenced phenotypic changes in both strains. There were populations of cells that increased with age in the PBMC and spleens of both strains (i.e., MHC class II+), decreased in the periphery and spleens of both strains (CD11b+) or did not change in the PBMC and spleens of both strains (CD8+). However, in many cases the age-related differences were genetically determined, strongly supporting the evidence of intrinsic connection between genetic background and ageing.