A Trypanosoma brucei bloodstream mutant in which both copies of the
ornithine decarboxylase (ODC) gene were knocked out (ODC mutant) was used to determine the
biological functions of ODC in T. brucei. Growth of the mutant cells ceased within 12-24 h in regular culture medium deficient in
polyamines, but could be rescued by supplementation with 1 mM
putrescine. A mouse model of T. brucei
infection was used to determine whether the mutant was still infective and was found to develop either extremely low or undetectable levels of
parasitemia, suggesting that in T. brucei, ODC activity is essential for establishing an
infection. Furthermore, when these mice were subsequently challenged with wild-type T. brucei cells expressing the same variant
surface glycoprotein (VSG), they did not develop any
parasitemia, indicating that inoculating the mice with the attenuated ODC mutant had conferred protection against challenge by wild-type cells. These results were reproduced in C57BL/6J mice deficient in alpha-beta and
gamma-delta T-cell receptors. However, no protection was observed in rag-2 knockout mice deficient in both B and T lymphocytes or in C57BL/10J mice deficient only in B lymphocytes. The results thus suggest that the ODC mutant could induce a T-lymphocyte-independent but B-lymphocyte-dependent immunity against wild-type cells of the same VSG. Such a mechanism of immunity has been elicited only by live T. brucei cells, but not by isolated VSGs or radiation-killed trypanosomes. This ODC mutant may thus represent a genuinely attenuated T. brucei bloodstream form capable of immunizing mammals against
infections by African trypanosomes of the same VSG subtype without causing detectable
infection by itself. The observation also raises the interesting likelihood that the in vivo treatment of T. brucei bloodstream forms with alpha-DL-
difluoromethylornithine is a de facto attenuation of the parasitic organisms, which may very well result in B-lymphocyte-dependent host immune responses to subsequent
infections by parasites of the same VSG subtypes.