The circumsporozoite protein (CSP), the major surface coat protein of the malaria sporozoite, 54 has been at the forefront of vaccination studies -being the basis of RTS,S/AS01, the most 55 progressed malaria vaccine candidate to date (9). Immunisation of BALB/c mice 56 with Plasmodium berghei (Pb) or P. yoelii (Py) RAS evokes immunodominant major 57 histocompatibility complex class I (MHC-I) H-2-K d -restricted CD8 + T cell responses against 58 distinct CSP epitopes: SYIPSAEKI for Pb (10) and SYVPSAEQI for Py (11). Indeed, the 59 measurement of responses to these epitopes has become the standard in fundamental 60 immunological studies in BALB/c mice (12) (13) (14) . Furthermore, numerous vaccination studies 61 involving different viral-vectored CSP-or CSP epitope-expressing vaccines -used alone or in 62 combination as part of prime-boost regimens -have corroborated that CSP is a highly 63 protective antigen in the BALB/c infection model (12) (13) (14) (15) (16) (17) (18) . In these studies, elevated levels of 64 either SYIPSAEKI-or SYVPSAEQI-specific CD8+ T cell responses correlated with protection.
Several studies have interrogated and contested the immunological relevance of CSP in 67 parasite-induced protection. These studies emanated from observations that in naturally 68 exposed humans T cell responses to CSP are scarce ( 
In this study, we have extended previous work on the entire CSP by dissecting the relevance 79 of a single CSP-derived immunodominant epitope in parasite-and vaccine-induced protection.
As the most stringent model system, we utilised transgenic Pb parasites lacking SYIPSAEKI 
Sporozoite-induced SYIPSAEKI-specific CD8+ T cell responses contribute to parasite 88 killing but are dispensable for the development of sterile immunity.
First, we interrogated the role that SYIPSAEKI, the H-2-K d -restricted immunodominant epitope 90 of PbCSP, plays in protective immunity induced after live attenuated sporozoite immunisation. 
To ascertain whether SYIPSAEKI contributes to parasite-induced protection, BALB/c mice 105 were immunised once with either PbWT or PbCSP SIINFEKL RAS. Two weeks after immunisation, 106 the mice were challenged with PbWT sporozoites and protection was determined by 107 measuring the parasite loads in the liver 40 hours later (Fig. 1B) . A significant reduction in 108 parasite load -up to four orders of magnitude difference as compared to naïve mice -was 109 observed in mice immunised with PbWT RAS and challenged with PbWT parasites. In contrast, 
To further investigate the requirement of SYIPSAEKI as the indispensable protective epitope 225 of CSP, mice unable to present this epitope were vaccinated with the PbCSP prime-boost 226 regimen with an interval of two weeks between vaccines, followed by challenge with either 227 PbWT or PbCSP SIINFEKL parasites (Fig 3A) . C57BL/6 mice were used because SYIPSAEKI is 228 an H-2-K d restricted epitope, and this mouse strain does not express the relevant MHC-I allele.
Thus, SYIPSAEKI would fail to be presented by infected hepatocytes. As before, blood and 230 serum were derived two weeks after boost. As expected, SYIPSAEKI-specific CD8+ T cells 
RTS,S/AS01, the leading subunit malaria vaccine based on CSP, seems to offer some 303 protection against P. falciparum re-infection (9). Partial and short-lived protection is likely 304 primarily mediated by the action of transitory anti-sporozoite antibodies (30-32). Strikingly,
peripheral blood CD8+ T cell responses were not identified to provide a role following 306 sporozoite challenge in this candidate vaccine. Together with previous findings (7, 14, 16, 21) 307 our data underscore efforts to improve the most advanced candidate malaria vaccine, 308 RTS,S/AS01, by eliciting CD8+ T cells against CSP or other immunodominant antigens. 
Immunofluorescent antibody assay. 10,000 sporozoites were spotted onto epoxy coated 343 glass slides with marked rings (Medco), dried at room temperature and stored at -20°C.
Thawed slides were fixed in acetone, dried and rehydrated with PBS before incubation in 10% 345 FCS supplemented DMEM (Gibco) for 1 hour at 37°C in a humid chamber. Serum at 346 concentrations 1:10 3 , 1:3.3x10 3 , 1:10 4 , 1:3.3x10 4 , 1:10 5 (and, additionally, 1:3.3x10 5 and 1:10 6 347 for C57BL/6 serum) were added to the ring wells and incubated for 1 hour at 37°C in a humid 348 chamber. Slides were washed and stained with a mouse anti-CSP (35) primary antibody.
Hoechst33342 was added as the nuclear stain together with a respective fluorescently labelled 
Peripheral blood was drawn from the tail vein and collected in Na + heparin capillary tubes 357 (Brand) and assayed in 96-well flat bottom plates (Corning). Red blood cells were lysed using 358 PharmLyse (BD) and lymphocytes were resuspended in 10% FCS, 2% Penicillin-Streptomycin 359 and 1% L-glutamine supplemented RPMI 1640 (Gibco). Splenocytes were counted using a 360 40x dilution with Trypan Blue (ThermoFisher Scientific) and a Neubauer 'Improved' 361 haemocytometer (Biochrom). 2x10 6 splenocytes and the lysed blood samples were prepared 362 in 96 well plates and incubated with a final concentration of 10µg/ml of SYIPSAEKI peptide in 363 in the presence of Brefeldin A (eBioScience) for 5-6 hours at 37°C and 5% CO 2 . For staining 364 of cell surface markers and intracellular cytokines, cells were incubated for 1 hour at 4°C for 365 each staining. Cells were stained for CD8 (53-6.7) and CD11a (M17/4) (eBiosience). Splenic 366 cells were fixed with 4% paraformaldehyde and peripheral blood cells were fixed with 1% 20 paraformaldehyde before staining for IFN-g (XMG1.2) (eBioscience) in the presence of 368 Perm/Wash buffer (BD) for intracellular staining. Data was acquired by flow cytometry using 369 an LSRFortessa or LSRII (BD) and analysed using Flowjo9.5.2 (Tree Star, Inc.).
Quantification of parasite load in the liver. Livers were harvested 40-42 hours after 372 sporozoite challenge and total RNA was extracted following homogenisation using TRIzol 373 (ThermoFisher Scientific). cDNA was generated using the RETROScript Kit (Ambion). 