Restoration of T-cell responsiveness by thymosin: development of antituberculous resistance in BCG-infected animals.

T-cell-depleted (adult thymectomized, lethally irradiated, bone marrow-reconstituted [THXB]), sham-thymectomized (XB) and normal control mice were injected daily with 3 mg of calf thymosin for 16 days. On day 8 of the treatment, the mice, together with untreated controls, were infected intravenously with 4 X 10(6) viable Mycobacterium bovis (BCG Montreal). Growth of the BCG in the lungs and spleens was compared quantitatively for up to 100 days. Thymosin treatment reversed the progressive weight loss seen in BCG-infected THXB mice and prevented their death due to the ongoing mycobacteriosis that developed in the T-cell-depleted animal. There was a late-developing anti-mycobacterial response in the thymosin-treated THXB mice, which resulted in a progressive decline in viability for the lung and spleen populations over the 40- to 80-day period, when the corresponding counts for the untreated THXB mice remained relatively constant. The histopathology of the lung and the increased antibacterial activity seen in the thymosin-treated THXB mice correlated with decreased [3H]deoxyribonucleic acid levels seen in the lungs and spleen compared with that present in the T-cell-depleted controls.