REACTIVATION OF LATENT LEISHMANIASIS BY INHIBITION OF INDUCIBLE NITRIC-OXIDE SYNTHASE

Nitric oxide (NO) generated by the inducible isoform of NO synthase (i NOS) is required for the resolution of acute cutaneous leishmaniasis i n resistant C57BL/6 mice. As is the case in several other infections, the clinically cured host organism still harbors small amounts of live Leishmania major parasites. Here, we demonstrate lifelong expression of iNOS at the site of the original skin lesion and in the draining ly mph node of long-term-infected C57BL/6 mice. iNOS activity in the lymp h node was dependent on CD4(+), but not on CD8(+) T cells. By double l abeling techniques, iNOS and L. major were each found in macrophages ( F4/80(+), BM-8(+), and/or MOMA-2(+)) and dendritic cells (NLDC-145(+)) , but not in granulocytes or endothelial cells. In situ triple labelin g of lymph node sections revealed that similar to 30-40% of the L. maj or foci were associated with iNOS-positive macrophages or dendritic ce lls. The majority of the L. major foci (60-70%), however, was located in areas that were negative for both iNOS and the macrophage and dendr itic cell markers. In L. major-infected C57BL/6 mice, which had cured their cutaneous lesions, administration of L-N-6-iminoethyl-lysine (L- NIL), a potent inhibitor of iNOS, led to a 10(4)-10(5)-fold increase o f the parasite burden in the cutaneous and lymphoid tissue and caused clinical recrudescence of the disease. Persistent expression of iNOS a nd resumption of parasite replication after application of L-NIL was a lso observed in resistant C3H/HeN and CBA/J mice. We conclude that iNO S activity is crucial for the control of Leishmania persisting in immu nocompetent hosts after resolution of the primary infection. Failure t o maintain iNOS activity might be the mechanism underlying endogenous reactivation of latent infections with NO-sensitive microbes during ph ases of immunosuppression.