Effects of a low-pathogen environment on natural killer cells of normal and B lymphocyte-deficient mice

The effect of short-term exposure (1 month) to a low-pathogen environment (LPE) on NK cell levels in the bone marrow and spleen of immunologically normal CBA/CaJ and B-lymphocyte-deficient CBA/N mice was assessed using both functional (51Cr release) and histological methods. The total NK activity (TNKA), i.e. the percentage specific lysis corrected for changes in whole organ cellularity, of the spleens of LPE-exposed CBA/CaJ mice was not significantly different from that of conventionally reared control mice of that strain (112%), while TNKA of the bone marrow of LPE-exposed mice fell to 27% of that of the bone marrow of conventionally reared controls. TNKA of the spleen and bone marrow of LPE-exposed CBA/N mice was reduced (30%) and elevated (120%), respectively, relative to their conventionally reared counterparts. The numbers of asialo-GM-1-positive (ASGM-1+) lymphoid cells, putative NK cells, in CBA/CaJ spleens were more numerous in conventionally reared than in LPE-exposed mice (4.9 X 10(6) vs. 2.7 X 10(6), and were also more numerous in the bone marrow of conventionally reared mice than in LPE-exposed animals (8.0 X 10(4), vs. 3.0 X 10(4) cells). Similarly, in LPE-exposed CBA/N mice, the numbers of ASGM-1+ lymphoid cells in the spleens and bone marrow were lower (2.7 X 10(6) and 5.4 X 10(4), respectively) than those of the spleens and bone marrow of their conventionally reared counterparts (3.8 X 10(6) and 10.0 X 10(4), respectively). The results demonstrate that short-term maintenance in an LPE affected the NK cells of both the spleen and bone marrow of immunologically normal and B-lymphocyte-deficient mice in a strain-specific manner and suggest that the external environment may regulate NK cell production and turnover.