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Prolonged antigen presentation is required for optimal CD8+ T cell responses against malaria liver stage parasites.

Cockburn IA, Chen YC, Overstreet MG, Lees JR, van Rooijen N, Farber DL, Zavala F - PLoS Pathog. (2010)

Bottom Line: Firstly, reducing the time primed CD8+ T cells were exposed to antigen in vivo severely reduced the final size of the developing memory population.Secondly, fully developed memory cells expanded in previously immunized mice but not when transferred to naïve animals.Finally, persisting antigen was able to prime naïve cells, including recent thymic emigrants, to become functional effector cells capable of eliminating parasites in the liver.

View Article: PubMed Central - PubMed

Affiliation: Johns Hopkins Malaria Research Institute and Department of Molecular Microbiology and Immunology, Bloomberg School of Public Health, Baltimore, Maryland, USA.

ABSTRACT
Immunization with irradiated sporozoites is currently the most effective vaccination strategy against liver stages of malaria parasites, yet the mechanisms underpinning the success of this approach are unknown. Here we show that the complete development of protective CD8+ T cell responses requires prolonged antigen presentation. Using TCR transgenic cells specific for the malaria circumsporozoite protein, a leading vaccine candidate, we found that sporozoite antigen persists for over 8 weeks after immunization--a remarkable finding since irradiated sporozoites are incapable of replication and do not differentiate beyond early liver stages. Persisting antigen was detected in lymphoid organs and depends on the presence of CD11c+ cells. Prolonged antigen presentation enhanced the magnitude of the CD8+ T cell response in a number of ways. Firstly, reducing the time primed CD8+ T cells were exposed to antigen in vivo severely reduced the final size of the developing memory population. Secondly, fully developed memory cells expanded in previously immunized mice but not when transferred to naïve animals. Finally, persisting antigen was able to prime naïve cells, including recent thymic emigrants, to become functional effector cells capable of eliminating parasites in the liver. Together these data show that the optimal development of protective CD8+ T cell immunity against malaria liver stages is dependent upon the prolonged presentation of sporozoite-derived antigen.

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Related in: MedlinePlus

Cells primed weeks after immunization can inhibit parasite development.Two groups of mice were immunized with 3×104 irradiated sporozoites i.d. in the right ear on d0 and depleted of the endogenous CD8+ T cell response on days 3 and 4. Some of these mice received 2×103 transgenic cells on d14 (white bar) while others did not receive cells (gray bar). Naïve control mice were similarly depleted of CD8+ T cells on days 3 and 4 and received transgenic cells (black bar). All mice were then challenged on d21 and euthanized 40 hours later. Parasite burdens in the liver were measured by RT-PCR (mean ± SE, * = P<0.05, ** = P<0.01, data from one of two experiments shown).
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ppat-1000877-g009: Cells primed weeks after immunization can inhibit parasite development.Two groups of mice were immunized with 3×104 irradiated sporozoites i.d. in the right ear on d0 and depleted of the endogenous CD8+ T cell response on days 3 and 4. Some of these mice received 2×103 transgenic cells on d14 (white bar) while others did not receive cells (gray bar). Naïve control mice were similarly depleted of CD8+ T cells on days 3 and 4 and received transgenic cells (black bar). All mice were then challenged on d21 and euthanized 40 hours later. Parasite burdens in the liver were measured by RT-PCR (mean ± SE, * = P<0.05, ** = P<0.01, data from one of two experiments shown).

Mentions: To determine if persistent antigen primed cells could develop anti-parasite acitivity we transferred transgenic cells to mice that had been immunized 14 days previously. Mice that received transgenic cells had been depleted of CD8+ cells on days 3 and 4 after immunization, to ensure that the only sporozoite specific effector CD8+ cells present in these mice would have been primed by persisting antigen. Seven days after cell transfer the mice were challenged with live P. yoelii sporozoites and the parasite load measured in the liver. Compared to naïve control mice - which had also been depleted of CD8+ T cells and received transgenic cells – parasite development in immunized mice was inhibited by approximately 50%. This level of anti-parasite activity was significant (P = 0.0016 by two-tailed T test; t = 4.96; df = 7), demonstrating that CD8+ cells primed by persisting antigen can develop anti-parasite effector mechanisms and are likely to contribute to protective immunity (Figure 9). Importantly, parasite killing could not be attributed to antibodies, CD4+ T cells or CD8+ T cells that survived depletion, since immunized mice that were treated with anti-CD8 but did not receive transgenic cells were infected just as readily as naïve control animals (Figure 9).


Prolonged antigen presentation is required for optimal CD8+ T cell responses against malaria liver stage parasites.

Cockburn IA, Chen YC, Overstreet MG, Lees JR, van Rooijen N, Farber DL, Zavala F - PLoS Pathog. (2010)

Cells primed weeks after immunization can inhibit parasite development.Two groups of mice were immunized with 3×104 irradiated sporozoites i.d. in the right ear on d0 and depleted of the endogenous CD8+ T cell response on days 3 and 4. Some of these mice received 2×103 transgenic cells on d14 (white bar) while others did not receive cells (gray bar). Naïve control mice were similarly depleted of CD8+ T cells on days 3 and 4 and received transgenic cells (black bar). All mice were then challenged on d21 and euthanized 40 hours later. Parasite burdens in the liver were measured by RT-PCR (mean ± SE, * = P<0.05, ** = P<0.01, data from one of two experiments shown).
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2865532&req=5

ppat-1000877-g009: Cells primed weeks after immunization can inhibit parasite development.Two groups of mice were immunized with 3×104 irradiated sporozoites i.d. in the right ear on d0 and depleted of the endogenous CD8+ T cell response on days 3 and 4. Some of these mice received 2×103 transgenic cells on d14 (white bar) while others did not receive cells (gray bar). Naïve control mice were similarly depleted of CD8+ T cells on days 3 and 4 and received transgenic cells (black bar). All mice were then challenged on d21 and euthanized 40 hours later. Parasite burdens in the liver were measured by RT-PCR (mean ± SE, * = P<0.05, ** = P<0.01, data from one of two experiments shown).
Mentions: To determine if persistent antigen primed cells could develop anti-parasite acitivity we transferred transgenic cells to mice that had been immunized 14 days previously. Mice that received transgenic cells had been depleted of CD8+ cells on days 3 and 4 after immunization, to ensure that the only sporozoite specific effector CD8+ cells present in these mice would have been primed by persisting antigen. Seven days after cell transfer the mice were challenged with live P. yoelii sporozoites and the parasite load measured in the liver. Compared to naïve control mice - which had also been depleted of CD8+ T cells and received transgenic cells – parasite development in immunized mice was inhibited by approximately 50%. This level of anti-parasite activity was significant (P = 0.0016 by two-tailed T test; t = 4.96; df = 7), demonstrating that CD8+ cells primed by persisting antigen can develop anti-parasite effector mechanisms and are likely to contribute to protective immunity (Figure 9). Importantly, parasite killing could not be attributed to antibodies, CD4+ T cells or CD8+ T cells that survived depletion, since immunized mice that were treated with anti-CD8 but did not receive transgenic cells were infected just as readily as naïve control animals (Figure 9).

Bottom Line: Firstly, reducing the time primed CD8+ T cells were exposed to antigen in vivo severely reduced the final size of the developing memory population.Secondly, fully developed memory cells expanded in previously immunized mice but not when transferred to naïve animals.Finally, persisting antigen was able to prime naïve cells, including recent thymic emigrants, to become functional effector cells capable of eliminating parasites in the liver.

View Article: PubMed Central - PubMed

Affiliation: Johns Hopkins Malaria Research Institute and Department of Molecular Microbiology and Immunology, Bloomberg School of Public Health, Baltimore, Maryland, USA.

ABSTRACT
Immunization with irradiated sporozoites is currently the most effective vaccination strategy against liver stages of malaria parasites, yet the mechanisms underpinning the success of this approach are unknown. Here we show that the complete development of protective CD8+ T cell responses requires prolonged antigen presentation. Using TCR transgenic cells specific for the malaria circumsporozoite protein, a leading vaccine candidate, we found that sporozoite antigen persists for over 8 weeks after immunization--a remarkable finding since irradiated sporozoites are incapable of replication and do not differentiate beyond early liver stages. Persisting antigen was detected in lymphoid organs and depends on the presence of CD11c+ cells. Prolonged antigen presentation enhanced the magnitude of the CD8+ T cell response in a number of ways. Firstly, reducing the time primed CD8+ T cells were exposed to antigen in vivo severely reduced the final size of the developing memory population. Secondly, fully developed memory cells expanded in previously immunized mice but not when transferred to naïve animals. Finally, persisting antigen was able to prime naïve cells, including recent thymic emigrants, to become functional effector cells capable of eliminating parasites in the liver. Together these data show that the optimal development of protective CD8+ T cell immunity against malaria liver stages is dependent upon the prolonged presentation of sporozoite-derived antigen.

Show MeSH
Related in: MedlinePlus