Induction of monocyte procoagulant activity by murine hepatitis virus type 3 parallels disease susceptibility in mice

To determine the kinetics of MHV-3-induced amplification of PCA, 1 X 106 PBM from BALB/c, G3H/St, and A/J mice were each incubated with 106 PFU of MHV-3 for various time intervals and assayed for total cellular PCA. A 10-fold PCA response was observed for BALB/c PBM that were cultured for 1 h with MHV-3 (Fig. 4) . There was a progressive increase of PGA with time of exposure, which reached a maximum of 84,500 mU at 12 h. This level of PGA was maintained for 48 h, which was the maximum interval of observation. In contrast, the PCA response of C3H/St mice did not rise until 2 h after exposure to MHV-3 and was maximum at 8 h (Fig. 4) at 2,800 mU, which was a 6.8-fold increase over control. Finally, there was no increase in PCA from A/J mice above a background level of 385 mU at any time interval of cultivation (Fig. 4) . from either C3H/St or BALB/c mice were directly stimulated with MHV-3, no increase of cellular PCA was observed (Table IV) . However, when PBM were first stimulated by MHV-3 for 18 h and then subsequently fractionated into lymphocytes and monocytes by adherence to plastic for 48 h, the increase in PCA observed for the whole PBM was localized to the monocyte population. This is similar to the increase demonstrated for LPS stimulation (14) . The PCA response of PBM from BALB/c mice was 168-fold, increasing from 505 to 81,400 mU. 98% of the increase in PCA could be accounted for in the monocyte fraction, which demonstrated an increase in total cellular PCA from 3,200 (control) to 722,000 mU/10 ~ monocytes (Table V) . were prepared from C3H/St, A/J, and BALB/c mice. They were stimulated either directly by MHV-3 alone attached in the 16-mm plastic wells or in the presence of variable numbers of lymphocytes, i.e., with lymphocyte:monocyte ratios from 0:1 to 8:1. Monocytes were then isolated from lymphocytes by vigorous washing and assayed for total cellular PCA. Monocytes (1 × 10 5) exposed to 10 6 PFU of MHV-3 in the absence of lymphoeytes exhibited no PCA response, nor was the response at lymphocyte:monocyte ratios of less than 2:1 particularly notable (Fig. 5) . However, at lymphocyte'monocyte ratios of 2.5:1 or greater, a significant PCA response was observed and it reached maximum levels as the ratio approached 4:1. No further increase in PCA was noted as the ratio was increased to 8:1. These data indicate that monocytes are the cellular source of the MHV-3-induced PCA, but the presence of lymphocytes is necessary for induction of the PCA response in monocytes.