L’anticorps monoclonal anti-CD20 (Rituximab) est largement utilisé dans le traitement des maladies auto-immunes. L’analyse de la rate des patients souffrant d’un purpura thrombopénique (PTI) ou d’une anémie hémolytique auto-immune traités par anti-CD20 a mis en évidence que la déplétion lymphocytaire B favorisait la différenciation des plasmocytes (PC) normaux en plasmocytes à longue durée de vie (PLDV) auto-réactifs, expliquant en partie l’absence de réponse à ce traitement. L’enjeu de ce projet a été de savoir si la déplétion lymphocytaire B induit l’émergence de PLDV spléniques et de comprendre les processus impliqués dans la survie plasmocytaire. Pour ce faire, nous avons utilisé le modèle de souris transgénique AID-Cre-ERT2xRosa26-loxP-EYFP qui permet de marquer irréversiblement par la protéine EYFP les cellules B lors de leur passage dans un centre germinatif au cours d’une réponse immune après ingestion de tamoxifène, puis de les suivre in vivo. Les PC EYFP+ ont été générés suite à 2 immunisations avec des globules rouges de mouton. Après avoir sélectionné un set de gènes permettant d’établir les signatures plasmablastiques et plasmocytaires, nous avons comparé par RT-PCR multiplex sur cellules uniques le profil d’expression des PC EYFP+ de la rate de souris traitées ou non par anti-CD20. Nous avons ainsi caractérisé dans le contexte de déplétion B une population plasmocytaire dans la rate homogène et mature, proche des PLDV de la moelle osseuse. Ce profil était différent de celui retrouvé dans la rate des souris contrôles, plus hétérogène, comprenant une majorité de PC intermédiaires entre plasmablastes et PC matures. Nous avons observé le même processus de différenciation paradoxale plasmocytaire dans la rate sous anti-CD20 dans le modèle murin lupique NZB/W, signifiant probablement un mécanisme général, que ce soit en contexte auto-immun ou non, chez l’homme et chez la souris. Nous avons identifié le BAFF (B-cell activating factor) comme un facteur essentiel dans le processus de survie des PC de la rate dans le contexte de déplétion B. En effet, le taux de BAFF augmente dans le sérum et le tissu splénique après traitement par anti-CD20, la combinaison in vivo des traitements anti-CD20 et anti-BAFF induit une diminution drastique des PLDV de la rate, sans générer d’hypogammaglobulinémie IgG. Les granuleux Gr1+ et en particulier les neutrophiles Ly6G+ semblent être la principale source de production de BAFF dans le contexte de déplétion B. Nous avons observé un effet similaire de la combinaison anti-CD20 et anti-BAFF sur les PLDV de la rate dans le modèle lupique NZB/W. Enfin, les LT CD4+ sont un autre composant important de la niche splénique dans le contexte de déplétion B. En effet, le nombre de PC EYFP+ diminue significativement avec l’association anti-CD20 et anti-CD4. Ces résultats suggèrent donc que l’association du traitement anti-CD20 à un inhibiteur de facteur de survie plasmocytaire spécifique de la rate, en particulier BAFF, pourrait avoir un bénéfice clinique au cours des maladies auto-immunes en interférant sur le processus paradoxal de maturation des PC. Un essai clinique associant les traitements anti-CD20 et anti-BAFF au cours du PTI débutera prochainement. / Previous data suggested that the monoclonal anti-CD20 antibody induced paradoxically the settlement of autoreactive splenic long-lived plasma cells (LLPC) in the spleen of patients with auto-immune cytopenia, explaining the treatment failure. To investigate whether this process had a general relevance and decipher its mechanism, we used the AID-CreERT2-EYFP mouse model, which allows the irreversible expression of EYFP in B cells engaged in an immune response after tamoxifen regimen to follow plasma cells at different times after immunization. When analyzed by multiplex PCR at the single-cell level, while the splenic EYFP+B220-PC of untreated mice displayed an intermediate profile between short-lived and long-lived PC, the PC from anti-CD20 treated mice composed a more mature homogeneous population, similar to the long-lived bone marrow PC. The absolute number of splenic EYFP+B220-PC did not change significantly upon anti-CD20 treatment indicating that B-cell depletion promoted PC differentiation rather than a long-lived PC selection. BAFF (B-cell activating factor) and CD4+ T-cells played a major role in plasma cell survival since combination of anti-CD20 with anti-BAFF or anti-CD4 antibodies dramatically reduced the number of splenic EYFP+B220- LLPC. Anti-CD20 treatment also promoted the differentiation of LLPC in the spleen in the lupus prone NZB/W model, while a treatment combining anti-CD20 with anti-BAFF induced a marked reduction in total splenic PC numbers. These results suggest that the process of PC maturation upon anti-CD20 treatment is a general mechanism and that interfering with anti-BAFF antibody at the time of B-cell depletion might greatly improve the response rate in auto-immune disease.

Malaria, caused by the apicomplexan parasite Plasmodium, is a major cause of morbidity and mortality throughout the world. This study has focused on the role of the spleen in the control of the blood stage of infection. Three aspects have been examined specifically: the effect of infection on the architecture of the spleen, the role of the spleen in parasite clearance and the formation of B cell memory. Firstly, the effect of infection on the splenic microarchitecture was examined. An essential component of the splenic architecture is the marginal zone (MZ), an area of the spleen that separates the reticuloendothelial red pulp of the spleen from the lymphoid white pulp compartment. Two unique populations of macrophages are found in the marginal zone: marginal zone macrophages (MZM) and marginal metallophilic macrophages (MMM). In the current study, parasitised red blood cells (pRBC) as well as normal RBC located to the MZ thirty minutes after intravenous injection and formed close associations with both MMM and MZM. Eight days after infection, at the time of peak parasitemia, a complete loss of both MMM and MZM was observed. Assays to detect cell death revealed that the loss of both MMM and MZM appeared to occur as a result of apoptosis. The apoptosis was not induced by up regulation of the inflammatory cytokines tumour necrosis factor or interferon-γ and could not be blocked by over expression of the apoptosis inhibitor Bcl2. Significantly, MMM were retained in the absence of CD8+ T cells implicating CD8+ T cells in the loss of MMM. Finally, infection of CD95-/- mice demonstrated that CD95/CD95-ligand (Fas/Fas-ligand) interactions were responsible for some of the CD8+ T cell-mediated loss of MMM. These data provide evidence for a novel interaction between MMM and CD8+ T cellsfollowing infection with Plasmodium. Secondly, the role of the spleen in the control of parasitemia and disease was monitored with an emphasis on determining the role of splenic macrophage populations (MMM, MZM and red pulp macrophages [RPM]) in parasite clearance. A clodronate liposome-mediated macrophage depletion technique was used, and caused a complete loss of all three macrophage sub-populations, as well as 50% of splenic dendritic cells, within 24 hours of administration. Each of the macrophage populations, as well as splenic DC, demonstrated different repopulation kinetics following their depletion from the spleen and these kinetics were utilised to examine each cell population in isolation. RPM depleted mice had significantly higher peak parasitemias than the controls. This peak returned to the level observed in undepleted control animals only after the repopulation of RPM was complete, suggesting that RPM play a role in the control of peak parasitemia following infection. Neither MMM nor MZM played a role in the control of parasitemia. The role of non-splenic macrophages and splenic dendritic cells also was investigated and shown to be insignificant in the absence of splenic macrophages. Finally, the role of RPM in mice immune to infection was investigated and their role shown to be dispensable, with immune mice clearing parasitemia efficiently in the absence of RPM. RPM therefore are important for the innate control of infection with P. chabaudi but are dispensible once adaptive immunity is established. Finally, the role of the spleen in the development of parasite-specific B cell memory was examined. Initial studies demonstrated that germinal centre (GC) development was compromised following infection with P. chabaudi, with an involution of B cell follicles noted early in infection. Adoptive transfer of memory B cells from immunised to naïve mice demonstrated that some protection was conferred on recipient mice by parasite-specific memory B cells. But, the memory B cells could not protect the host from developing parasitemia and did not produce significant amounts of parasite-specific immunoglobulin within seven days of challenge infection. Memory B cells could not be detected ten weeks after infection, indicating that the development, or survival, of parasite-specific memory B cells was compromised. The development of bystander memory B cells was not affected by infection. Finally, long-lived plasma cells were shown to develop in response to infection, although re-exposure of the cells to parasites in the form of recrudescent parasitemia resulted in their loss. This study therefore has identified a defect in the development of long-term, B cell-mediated, protection against infection with P. chabaudi. Each of these factors has significant implications for the understanding of how the spleen contributes to the control of infection with Plasmodium and potential applications for the further development of malaria vaccines and treatment regimens.