Attenuation of Virulence of M, lepraemurium after Serial Mouse Passages During Long Years

Abstract

Virulence comparison test between Hawaiian M (HM) and Hawaiian B (HB) strains was made on spread subcutaneous tissue preparations in male and female mice of C3H strain and male mice of BALB/c strain.Tested mice were inoculated subcutaneously in their back with 0.25ml of a1: 1, 000 leproma suspension (with HM or HB) and techniqe of the tissue spreading was described in our earlier report (5).In cases infected with HM bacilli, subcutaneous lesions developed gradully without reactional cell infiltration during the first 3 weeks after infection. Bacillus-containing mononuclears increased in number and became larger in size. At 3 weeks, in female C3H and male BALB/c mice, a small number of lymphocytes and a few polymorphonuc-lears were found on the specimens. After 5 weeks, lymphocytic infiltration with polymo-rphonuclears appeared around the bacillus-containing mononuclears (murine leprosy cells). Especially in male BALB/c mice, at 7 weeks, their subcutaneous lesions showed typically benign feature in which severe infiltration with lymphocytes and polymorphonuclears were observed, surrounding a small number of murine leprosy cells. At week 7, these cell reactions were occationally observed in male C3H mice.On the other hand, in almost all the cases HB bacill-induced lesions developed with time, without reactional cell infiltration, throughout the obsergation period, . After 5 weeks, the lesions showed typically malignant feature. Enlarged mononuclears, being crowded with long baiclli, were scattered or accumulated in the whole specimens. And in male BALB/c mice with HB bacilli a small number of lymphocytes appeared around these mononuclears at about 5 to 7 weeks. However, this feature was considered to be malignant so as to be non-typical.These results clearly showed that HM bacilli were much less virulent for these mice than HB bacilli.It is assumed from these findings that difference in virulence between HM and HB bacilli results from their ability to induce cell-mediated immunity in the host.Plate I Subcutaneous lesions, at week 3. (×200, Z-N stained)There are no pronounced differences in subcutaneous lesions between tested mice infected with HM and HB bacilli 3 weeks after infection.However, in female C3H and male BALB/c mice infected with HM bacilli, a small number of lymphocytes and a few polymorphonuclears are observed around murine leprosy cells (Photos 3 and 5). On the other hand, large and round-shaped murine leprosy cells are found without cell reactions in the remaining cases (Photos 1, 2, 4 and 6).Plate II Subcutaneous lesions, at week 7. (×200, Z-N stained)Remarkable differences are found in subcutaneous lesions between male BALB/c and female C3H mice infected with HM and HB bacilli, 7 weeks after infection. Severe lymphocytic infiltration with polymorphonuclears, surrounding murine leprosy cells, are found in male BALB/c mice with HM bacilli (Photo 11). This feature is typically benign. And there are numerous large murine leprosy cells in the specimens of male BALB/c with HB bacilli. However, very slight cell reaction appears around murine leprosy cells (Photo 12). Then, that feature is considered to be malignant but to be non-typical.The lesions of female C3H mice with HM bacilli (Photo 9) show similar tendencies to those of male BALB/c mice with HM bacilli. However, cell reactions are much more severe in the latter than in the former. And many large murine leprosy cells are observed in the remaining 3 groups without cell reactions (Photos 7, 8 and 10). These features are typically malignant.