「訓練」免疫系統的革命性 HIV 疫苗通過了它的第一個障礙
資料來源:Gus Cairns/2022 年 12 月 19 日/aidsmap/財團法人台灣紅絲帶基金會編譯
圖片來源:古斯塔沃·弗林/Pexels
一種旨在誘導「優於自然」的 HIV 反應的全新疫苗已完成其首次人體免疫原性試驗,並取得了令人鼓舞的結果。
該疫苗目前被稱為 eOD-GT8,由加州斯克里普斯研究所的 William Schief 教授及其同事與 Fred Hutchinson 癌症中心、國際愛滋病疫苗計畫、美國過敏和傳染病研究所合作開發, 和其他一些研究機構。 製藥公司 Moderna 參與了下一階段的試驗,因為將使用該公司在其 SARS-CoV-2 疫苗中使用的相同 mRNA 疫苗技術。
該疫苗旨在產生免疫細胞(B 淋巴細胞),在接觸 HIV 時,這些免疫細胞會用稱為 VRC01 的廣泛中和抗體 (broadly neutralising antibodies, bnAbs) 淹沒人體。 這種特殊的 bnAb 能夠中和(阻斷)HIV 包膜蛋白的一個非常特殊的部分,這是病毒進入 CD4 細胞所必需的。
問題是,到目前為止,bnAb 從未在人類身上被疫苗誘導,只有少數感染了慢性 HIV的人才有。 這是因為很少有 B 淋巴細胞具有從頭開始製造這些不常見的「超突變」bNAb 的內在能力。
Scripps 疫苗使用一種稱為「種系靶向」(germline targeting) 的技術來極大地擴大 B 淋巴細胞庫,這些 B 淋巴細胞具有能夠製造 bNAb 的必要基因,而目前的試驗 G001 已經成功做到了這一點——這是一項了不起的壯舉,本質上重新設計接受者部分的免疫系統。
eOD-GT8 疫苗旨在引發這種遺傳反應,而不是實際誘導 bNAb,並且不包含通常會引起抗體反應的 HIV 包膜蛋白的所有成分。 在未來的試驗中,希望透過使用與產生最強 bNAb 反應的 HIV 包膜蛋白片段更緊密結合的疫苗,將產生 VRC01 bNAb 的潛在能力變為現實。這些被稱為抗原表位 (Epitope)。
G001 研究的結果是同類研究中的第一個,發表在《科學》雜誌上。 在這項研究中,兩劑疫苗中較高劑量的疫苗誘導能夠產生 VRC01 bnAb 的細胞數量是自然產生的 550 倍,並且在應答者中,淋巴結中具有基因突變的 B 細胞百分比( 稱為 VRC01-2*02 或 VRC01-2*04)的能夠產生 bNAb 的細胞增加到 6.7%,而未接種疫苗的人的平均水平為 0.00023%(429,000 個細胞中只有一個細胞)。
與沒有 VRC01-2 突變的細胞相比,這些細胞表面產生的受體分子的親和力——它們對 HIV 表位或對這些表位的抗體的敏感性——增加了 840 倍。
更多相關的背景
有關為什麼 HIV 是一種如此難以開發 HIV 疫苗的病毒的更多詳細信息,請參閱此處,但核心原因可能是 HIV 能夠迅速變異而不受免疫控制。 在絕大多數感染病例中,病毒在免疫系統幾乎沒有離開起跑線之前就已經贏得了與免疫系統的比賽,有些人最終會產生廣泛中和的抗體,但為時已晚,因為病毒已經進化出戰勝他們的能力。 但是,如果我們能夠將這種能力注入足夠多的 B 淋巴細胞中,以便在它們在看到 HIV 的那一刻就產生針對 HIV 的 bnAb,那麼免疫系統就會贏得比賽。
Schief 團隊使用的方法並不是唯一嚐試這樣做的方法。 在稱為表位聚焦設計的方法中,研究人員嘗試開發一種多價疫苗,該疫苗可盡可能多的識別HIV 表位並產生針對它們的 bNAb。
以譜係為中心的設計對此進行了逆向工程。 科學家們研究了在慢性感染中發展出人體內的 bNAb,找出他們針對哪些 HIV 表位做出反應,然後設計疫苗來刺激針對這些表位的疫苗。
這兩種方法的問題在於,具有製造 bNAb 的基因的 B 淋巴細胞太少,以至於無法產生有效量的 bNAb。
種系靶向的作用是將逆向工程更進一步,並追問:我們能否設計一個進化的免疫過程,從不特異性識別任何錯誤的幼稚 B 淋巴細胞開始,並以記憶 B 細胞結束, 能產生大量不同的 bnAb 來應對 HIV 嗎?
VRC01 是最早在長期感染 HIV 的人的血液中發現的廣泛中和抗體之一。 它的結構和效力於 2010 年確定。在使用抗體輸注治療 HIV 或預防 HIV 感染的研究中,它具有一定的療效,但其本身不足以抑制 HIV 複製,其預防 HIV 的功效也很有限。 然而,擁有一個一旦檢測到 HIV 就準備好產生大量 VRC01 的免疫系統可能會更好。
Scripps 研究人員選擇 VRC01 作為他們希望生成的 bNAb 的原因不是因為它已經被使用過,而是因為能夠產生它的細胞具有獨特的遺傳特徵。 所有抗體都是 Y 形分子。 它需要產生成為 VRC01 抗體的特定系列氨基酸(蛋白質成分)的基因是 VRC01-2*02 或 *04 基因中的一個或兩個,它們產生抗體的「重鏈」( 一個臂和 Y 的莖幹)和任何五個,但不是任何其他數量的產生「輕鏈」的氨基酸,Y 的另一個臂。
超過 400,000 個 B 細胞中只有一個天然具有這種遺傳構象,感染後遇到的 HIV 包膜蛋白似乎不會刺激這些「前體」B 細胞特異性增殖,也不會產生 bNAb。
Schief 的團隊所做的是,根據之前在小鼠身上進行的臨床試驗的證據,設計一種疫苗,它會優先導致具有這種遺傳構象的細胞分裂和增殖。 它的設計目的不是,也沒有生產 VRC01 bNAb 本身,而是為了生產更多製造它的細胞。
這使得這項研究變得更加複雜:測量血液中抗體的數量是一個相對簡單的過程,但要查看製造它們的細胞中的基因要困難得多,研究人員不得不使用幾種方法,包括使用流式細胞術分選細胞以及使用 RNA 和 DNA PCR 檢測。
疫苗和研究參與者
eOD-GT8 疫苗是一種病毒樣顆粒——一種非人類蛋白質的空心外殼。 病毒樣顆粒疫苗並不新鮮——預防尖圭濕疣、子宮頸癌和肛門癌的 HPV 疫苗就是其中之一。 在這種情況下,非人類蛋白質是 lumazine 合酶,這是一種在生物體中發現的酶,可以製造自己的核黃素(維生素 B2)。 作為其合成功能的一部分,六十個酶分子自發地組裝成一個空心殼。 為此,研究人員附加了 60 個 HIV 包膜蛋白分子,每個酶單位一個,以在具有 VRC01 遺傳特徵的細胞中引發增殖。
這種疫苗,或者是安慰劑,間隔八週分兩劑分別給予 48 名志願者; 12 人接受了安慰劑,18 人接受了 20 微克的疫苗劑量,18 人接受了 100 微克的劑量。 在兩種疫苗劑量中加入 50 微克佐劑,這是一種可增強對疫苗免疫反應的化合物。 在這種情況下,佐劑是 ASO1B,這是兩種天然存在的化合物的混合物,可幫助疫苗將自身附著在 B 淋巴細胞受體(感知外來物質的表面蛋白)上。
志願者在性別(18 名女性,24 名男性)和出生時性別(其中四名男性是跨性別者;沒有女性是跨性別者,但兩名志願者稱自己是性別酷兒或不符合者)方面相當均衡。
這項美國研究在種族方面並不是那麼平衡; 33 人(69%)被歸類為白人,其中 6 人是拉丁裔; 四個是黑人,五個是亞洲人,三個是混血兒,一個是不知名的。 他們的平均年齡是 30 歲。
疫苗確實有一些副作用:與接受安慰劑的人相比,接受疫苗的人抱怨注射部位疼痛、輕度頭痛、發冷、關節痛、輕度發燒和不適或疲勞。 然而,嚴重症狀(3 級)的發生率並不高於安慰劑接受者。
疫苗的作用
該疫苗確實在不產生 bNAb 的幼稚 B 細胞和 T 細胞中產生了一般反應,顯示它確實具有一般免疫原性(副作用也顯示了這一點)。 血液中循環的 B 細胞增加了大約一千倍。 淋巴結中的 B 細胞在返回血流之前被反復暴露於外來抗原並轉變為識別特定抗原的記憶細胞,增加了 100,000 倍或更多。
但他們沒有產生任何針對愛滋病毒的反應。 這是預料之中的,因為這種啟動疫苗並非設計用於直接對 HIV 產生反應,並且不包含已知以這種方式具有直接免疫原性的 HIV 表面蛋白。
對 HIV env 蛋白的強烈的普遍免疫反應實際上會適得其反,因為它會轉移 B 細胞產生對 HIV 無效的非中和抗體。 關鍵是要擴大前體細胞的數量,其遺傳特徵顯示,如果使用含有特定免疫原性 HIV 表位的後期疫苗,所產生的有用部分抗體將是 bnAb。
該疫苗的主要目標是擴大能夠製造 bnAb 的 B 細胞數量,這一目標已經實現。 在這裡,研究人員沒有詳細說明,而是觀察了三個 B 淋巴細胞群,並計算了具有 VRC01 基因特徵的比例。 第二,在血液中循環的抗體和在淋巴結中成熟的抗體,會產生最常見的抗體類型,即免疫球蛋白 G (IgG),它佔循環抗體的 75%。 他們還測量了血液中產生一種罕見抗體免疫球蛋白 D 的細胞數量,這種抗體只佔抗體的 1%,但似乎對其他 B 細胞的抗體產生具有指導作用。
在疫苗接種前的樣本中,研究人員發現六名參與者可以識別出具有 VCR01 特徵的前體細胞,但實際上只有一兩個單獨的細胞,這顯示即使在這些參與者中,429,000 個 B 細胞中也只有一個具有這種特徵。 接種疫苗後,他們還在兩名安慰劑接受者身上發現了這種特徵。
在接種疫苗的參與者中,VRC01 細胞在除一名參與者外的所有參與者中增殖。 結果發現該參與者俱有不常見的 VRC01 遺傳特徵 VCR01-2*05/*06,顯示可能有 2%(主要是美國白人參與者)對該疫苗無反應。 來自世界其他地區的人是否會有不同的遺傳特徵是一個有待回答的問題。
第一次接種疫苗後 4 週,低劑量接受者中每 10,000 個循環血細胞中有一個和高劑量接受者中每 4,500 個循環血細胞中有一個具有產生 VRC01 的潛力,分別是接種疫苗前頻率的 43 倍和 94 倍。
到第 10 週,即第二次接種疫苗兩週後,低劑量接受者的 1,139 個 B 記憶細胞中有一個和高劑量接受者中的 777 個 B 記憶細胞中有一個屬於 VRC01 前體類型,分別是接種前頻率的 377 倍和 552 倍。 到第二次給藥後八週,這個數字分別下降到 3764 個細胞中的一個和 2019 個細胞中的一個。
需要強調的是,即使在高峰期,儘管現在除了一名參與者之外所有參與者的 B 細胞都帶有受體,表明它們在遇到 HIV 表位時可以產生 bnAb,但這仍然只佔所有記憶 B 細胞的 0.088%。 研究人員的希望是,既然已經誘導出這些前體細胞,那麼使用確實含有實際 HIV 表位(與這個不同)的疫苗進行進一步免疫現在將產生有效量的 VRC01 bNAb。
在淋巴結中成熟的 B 細胞中,只有約 60% 的參與者在其淋巴結中成熟的 VRC01 型 B 細胞數量有所增加。 然而,在那些的細胞中,具有 VRC01 基因特徵的細胞數量比以前多了約 100,000 倍,所有產生 IgG 的 B 細胞中高達 6.7% 具有 VRC01 基因特徵。
一個有希望的跡像是,在具有 VRC01 基因的細胞群中,其餘指導細胞產生抗體的基因存在很大差異。 這種「多克隆性」顯示它們應該能夠對多種 HIV 變體做出反應。
敏感基因測序還發現,即使在接種第二種疫苗後八週內 VRC01 細胞的絕對數量已經下降,但具有 VRC01 特徵的細胞中的突變數量仍在繼續積累,顯示製造各種 VRC01- 類型 bnAbs。
這些突變還使 VRC01 bnAb 生成細胞對典型的 HIV env 蛋白表位的敏感性增加,顯示不僅可能響應 HIV 暴露而產生的 bnAb 細胞數量增加,而且它們的敏感性也增加。 第一次接種疫苗三週後,B 細胞受體對 VCR01 型細胞中 HIV env 表位的親和力高 840 倍(這意味著 env 蛋白產生反應的能力比在非 VRC01 細胞中產生反應的能力低 840 倍) 在第二次之後的三週內,上漲了 32,400 倍。
這很重要,因為它顯示,即使最初產生 VRC01 的 B 細胞可能是少數群體,它們的 bNAb 產生也不會被大多數產生無效、非中和抗體的細胞所競爭,而 HIV 很容易從這些抗體中發生突變而遠離。
這可以變成真正的疫苗嗎?
在對研究論文的評論中,南非國家傳染病研究所的 Penny Moore 教授本人也是一位著名的 HIV 疫苗研究人員,她稱讚這是對一個非常複雜的概念的第一個證明。
她指出,這項研究需要在來自其他地區和種族的參與者中重複進行,並且還需要使用疫苗進行研究,這些疫苗能夠擴大能夠產生比 VRC01 更強大的其他 bnAb 的細胞群,並針對部分 HIV 包膜 CD4 結合位點以外的蛋白質。 這可能更加困難,因為其他 bnAb 源自具有較少特異性遺傳特徵的細胞。
然而,主要的挑戰在於,這只是該疫苗概念的第一步。 下一步將是依次為人們接種一系列疫苗,這些疫苗所含的表位更加接近實際的 HIV 病毒 env 蛋白。
如何做到這一點是有問題的。 含有大量 HIV 表位的疫苗可能必須具有高容量,並且還存在可能壓倒免疫系統並產生大量但非特異性反應的風險,這可能不僅不安全而且對 HIV 無效。 另一種方法是給人們提供一長串疫苗,旨在「引導」免疫系統產生 bnAb,但這在後勤和臨床上都具有挑戰性。
一個可能的答案是,在 G002 等試驗中,下一階段的種系靶向疫苗將使用基於 mRNA 的疫苗,而不是病毒樣顆粒。 這些將告訴細胞製造自己的顆粒,而不是直接注射。 這意味著可以將許多不同的說明包裝成更小劑量的疫苗,而且各種疫苗注射劑的生產應該更容易。 G003 試驗正在盧安達和南非進行,這可能會回答這樣一個問題:一種旨在引發 VRC01 的疫苗是否也適用於非洲人群。
從本質上講,Scripps 研究人員所做的是創造大量細胞,至少有可能製造一種特定類型的 bnAb,這在以前是微不足道的。 打個比方,在 HIV 與免疫系統的兔子與烏龜比賽中,這個想法是為烏龜配備噴氣背包,以便兔子發現時它已經在終點線等待了。
Schief 及其同事評論說,他們已經實現了「對免疫反應特異性的前所未有的控制」,這可能「預示著針對 HIV 和其他病原體的精確疫苗設計的新時代」。 換句話說,雖然將這種初免疫苗轉變為完整、有效的初免和加強疫苗過程可能仍然具有挑戰性,但它使用的科學方法對更複雜的疫苗設計具有普遍意義。
參考文獻:
Leggat DJ, Cohen KW, Schief WR et al. 疫苗接種在人類中誘導 HIV 廣泛中和抗體前體。 科學,378(6623):預印本,2022 年 12 月 2 日。www.doi.org/10.1126/science.add6502。
Moore PL. 透過接種疫苗觸發罕見的 HIV 抗體。 科學 378 (6623):949-950,2022 年 12 月 2 日。www.doi.org/10.1126/science.adf3722。
Revolutionary HIV vaccine that ‘trains’ the immune system passes its first hurdle
Gus Cairns/19 December 2022/aidsmap
Gustavo Fring/Pexels
A completely new kind of vaccine that aims to induce a response to HIV that is ‘better than nature’ has completed its first immunogenicity trial in humans, with promising results.
The vaccine, currently dubbed eOD-GT8, is being developed by Professor William Schief and colleagues at the Scripps Research Institute in California, in collaboration with the Fred Hutchinson Cancer Center, the International AIDS Vaccine Initiative, the US Institute of Allergies and Infectious Diseases, and a number of other research institutes. The pharmaceutical company Moderna are involved in the next stage of trials, as the same mRNA vaccine technology that the firm used in its SARS-CoV-2 vaccine will be used.
The vaccine aims to bring into being immune cells (B-lymphocytes) that will flood the body with broadly neutralising antibodies (bnAbs) of the type called VRC01 in the event of an exposure to HIV. This particular bnAb has the ability to neutralise (block) a very specific part of the HIV envelope protein that is necessary for the virus to enter CD4 cells.
The problem is that bnAbs have, so far, never been induced by a vaccine in humans, only by chronic HIV infection in a minority of people. This is because very few B-lymphocytes have the inbuilt capacity to ever make these uncommon, ‘hypermutated’ bNAbs from scratch.
The Scripps vaccine uses a technique called ‘germline targeting’ to greatly expand the pool of B-lymphocytes that have the necessary genes to be able to make bNAbs, and the current trial, G001, has managed to do this – a remarkable feat that essentially redesigns part of recipients’ immune systems.
The eOD-GT8 vaccine was designed to elicit this genetic response, not to actually induce bNAbs, and did not contain all the components of the HIV envelope protein that usually give rise to an antibody response. In future trials, it is hoped to turn the potential capability to produce VRC01 bNAbs into a reality by using vaccines ever more closely aligned to the subsections of the HIV envelope protein that give rise to the strongest bNAb responses. These are called epitopes.
Findings from the G001 study, the first of its kind, are published in Science. In the study, the higher of two doses of the vaccine induced the production of 550 times as many cells capable of producing the VRC01 bnAb as naturally occur, and in responders, the percentage of B-cells in lymph nodes that had the genetic mutations (called VRC01-2*02 or VRC01-2*04) that were capable of making bNAbs increased to 6.7%, compared with an average of 0.00023% (just one cell in 429,000) in unvaccinated people.
The affinity of the receptor molecules produced on the surface of these cells – their sensitivity to HIV epitopes or to antibodies to those epitopes – increased 840-fold in comparison to cells that did not have the VRC01-2 mutations.
More on the background
For more details on why HIV is such difficult virus to develop a vaccine against HIV, see this, but perhaps the core reason is HIV’s ability to rapidly mutate away from immune control. In the vast majority of cases of infection the virus has already won the race against the immune system before the latter is scarcely off the starting blocks.
Some people eventually develop broadly neutralising antibodies, but do so too late, when the virus has already evolved the ability to outwit them. But if we could infuse the capability into enough B-lymphocytes to produce bnAbs against HIV the moment they saw it – then the immune system would win the race.
The approach used by Schief’s team is not the only one that tries to do this. In the approach called epitope-focused design, researchers try to develop a multivalent vaccine that recognises as many HIV epitopes as possible and produces bNAbs against them.
Lineage-focused design reverse-engineers this. Scientists look at the bNAbs in people who develop them in chronic infection, work out what HIV epitopes they developed in reaction to, and then design vaccines to stimulate vaccines to those epitopes.
The problem with both these approaches is that there are so few B-lymphocytes that have the genes to make bNAbs that effective amounts are not generated.
What germline targeting does is to take the reverse-engineering one stage further back and ask: can we design an evolutionary immune process that starts with naïve B-lymphocytes that don’t specifically recognise any bug, and ends up with memory B-cells that can produce floods of varied bnAbs in response to HIV?
VRC01 was one of the first broadly neutralising antibodies to be discovered, in the blood of a person with long-term HIV infection. Its structure and potency were determined in 2010. In studies using infusions of the antibody to treat HIV or to prevent HIV infection, it had some efficacy but was in itself not sufficient to suppress HIV replication and its efficacy in preventing HIV was only modest. However having an immune system that is primed to produce large amounts of VRC01 as soon as it detects HIV may work better.
The reason the Scripps researchers picked VRC01 as the bNAb they wished to generate is not because it has already been used, but because the cells with the capability to produce it have a unique genetic signature. All antibodies are Y-shaped molecules. The genes that it needs to produce the particular series of amino acids (protein components) that becomes the VRC01 antibody are either one or both of the VRC01-2*02 or *04 genes, which produce the ‘heavy chain’ of the antibody (one arm and the stem of the Y) and any five, but not any other number, of amino acids that produce the ‘light chain’, the other arm of the Y.
Only one B-cell in over 400,000 naturally has this genetic conformation, and HIV envelope proteins encountered upon infection do not seem to stimulate these ‘precursor’ B-cells to specifically proliferate, nor to generate bNAbs.
What Schief’s team did was to design, on the evidence from previous clinical trials in mice, a vaccine that would preferentially cause cells with this genetic conformation to divide and proliferate. It was not designed to, and did not, produce the VRC01 bNAb itself, but to produce more of the cells that make it.
This made the study a lot more complicated to do: it is a relatively simple process to measure the amounts of an antibody in blood, but it’s much harder to look at the genes in the cells that make them, and the researchers had to use several ways to do this, including sorting cells with flow cytometry and using RNA and DNA PCR assays.
The vaccine and the study participants
The eOD-GT8 vaccine is a virus-like particle – a hollow shell of a non-human protein. Virus-like particle vaccines are not new – the HPV vaccine that prevents genital warts and cervical and anal cancers is one. In this case, the non-human protein is lumazine synthase, an enzyme found in organisms that can make their own riboflavin (vitamin B2). Sixty enzyme molecules spontaneously assemble into a hollow shell as part of its synthesis function. To this, the researchers attached 60 HIV envelope protein molecules, one per enzyme unit, attuned to elicit proliferation in cells with the VRC01 genetic profile.
This vaccine, or a placebo, was given to 48 volunteers in two doses eight weeks apart; 12 received placebo, 18 received a vaccine dose of 20 micrograms and 18 a dose of 100 micrograms. To both vaccine doses were added 50 micrograms of an adjuvant, a compound that amplifies an immune response to a vaccine. In this case the adjuvant was ASO1B, a mix of two naturally-occurring compounds that assist the vaccine to attach itself to B-lymphocyte receptors (surface proteins that sense foreign substances).
The volunteers were quite balanced in terms of gender (18 women, 24 men) and sex at birth (four of the men were trans; none of the women were, but two volunteers described themselves as genderqueer or non-conforming).
This US study wasn’t so balanced in term of race; 33 (69%) were classed as White of whom six were Latino; four were Black, five Asian, three of mixed race and one unknown. Their mean age was 30.
The vaccine did have some side effects: more people receiving the vaccine than people receiving placebo complained of pain at the injection site, mild headache, chills, joint pain, mild fever and malaise or fatigue. However the rate of severe symptoms (grade 3) was not higher than in placebo recipients.
The vaccine’s effects
The vaccine did produce a general reaction in non-bNAb-producing naïve B-cells and T-cells, showing it did have general immunogenicity (as also shown by the aside effects). B-cells circulating in the blood increased about a thousandfold. B-cells in the lymph nodes, where they are taken to be repeatedly exposed to foreign antigens and to turn into memory cells that recognise specific antigens, before being returned to the bloodstream, increased by a factor on 100,000 or more.
But they did not develop any HIV-specific responses. This was expected, as this priming vaccine was not designed to produce a reaction directly to HIV and did not contain HIV surface proteins known to be directly immunogenic in this way.
A strong generalised immune reaction to HIV env proteins would in fact have been counterproductive, as it would divert B-cells into producing non-neutralising antibodies against HIV that don’t work. The point was to magnify the population of precursor cells whose genetic signature showed that, if a later vaccine containing a specific immunogenic HIV epitope was used, a useful proportion of the antibodies produced would be bnAbs.
The main goal of the vaccine, to magnify the population of B-cells capable of making bnAbs, was achieved. Without going into exactly how here, the researchers looked at three populations of B-lymphocytes and counted the proportion that had the VRC01 genetic profile. Two, ones circulating in the blood and ones maturing in the lymph nodes, make the most common type of antibody, immunoglobulin G (IgG), which makes up 75% of circulating antibodies. They also measured the amounts of cells in the blood making a rare type of antibody, immunoglobulin D, which only forms 1% of antibodies but appears to have a role directing the antibody production of other B-cells.
In pre-vaccination samples, the researchers found six participants where precursor cells with the VCR01 profile could be identified, but literally in only one or two individual cells, indicating that even in these participants, only one in 429,000 B-cells had this signature. Post-vaccination, they also found this signature in two placebo recipients.
Among vaccinated participants, VRC01 cells proliferated in all but one participant. This participant turned out to have an uncommon genetic VRC01 signature, VCR01-2*05/*06, showing that maybe 2% (of predominantly White US participants) might be non-responders to this vaccine. Whether people from other parts of the world would have different genetic profiles is a question yet to be answered.
Four weeks after the first vaccination, one in 10,000 circulating blood cells in the lower-dose recipients and one in 4500 in the higher-dose recipients had VRC01-producing potentiality, representing respectively 43 and 94 times the pre-vaccination frequency.
By week 10, two weeks after the second vaccination, one in 1139 B-memory cells in low-dose recipients and one in 777 in high-dose ones were of the VRC01 precursor type, representing 377 and 552 times the pre-vaccination frequency. By eight weeks after the second dose this had gone down to one in 3764 and one in 2019 cells respectively.
It’s important to emphasise that even at peak, although all but one participant now had B-cells with receptors that indicated they could produce bnAbs when encountering HIV epitopes, this still only represented 0.088% of all memory B-cells. The hope of the researchers is that, now that these precursor cells have been elicited, further immunisations with vaccines that do contain actual HIV epitopes (unlike this one) will now produce effective amounts of the VRC01 bNAb.
In the B-cells maturing in lymph nodes, only about 60% of participants had an expansion in the number of VRC01-type B-cells maturing in their lymph nodes. Amongst those that did, however, the number of cells with the VRC01 gene signature was about 100,000 times greater than before, with up to 6.7% of all IgG-producing B-cells having the VRC01 gene signature.
A promising sign was that within the population of cells with the VRC01 genes, there was wide variation in the rest of the genes that instruct the cell what antibodies to make. This ‘polyclonality’ indicates that they should be able to respond to a wide variety of HIV variants.
Sensitive gene sequencing also found that, even while the absolute number of VRC01 cells was already declining within eight weeks of the second vaccine, the number of mutations in cells with the VRC01 signature continued to accumulate, indicating increased potentiality to make a variety of VRC01-type bnAbs.
These mutations also confer increased sensitivity of VRC01 bnAb-making cells to typical HIV env protein epitopes, indicating an increase not only in the amount of bnAb cells that might be produced in response to an HIV exposure, but also in their sensitivity. Three weeks after the first vaccine, the affinity of B-cell receptors to HIV env epitopes in VCR01-type cells was 840 times higher (meaning 840 times less env protein would produce a reaction than what would produce a reaction in non-VRC01 cells) and three weeks after the second, 32,400 times higher.
This is important because it suggests that, even though initially VRC01-producing B-cells might be a minority population, their bNAb production would not be outcompeted by the majority of cells that produce non-effective, non-neutralising antibodies which HIV could easily mutate away from.
Can this be turned into an actual vaccine?
In a commentary on the research paper, Professor Penny Moore of the South African National Institute for Communicable Diseases, herself a prominent HIV vaccine researcher, hails this as the first proof of a very complex concept.
She notes that the research needs to be repeated in participants from other regions and ethnicities, and also that studies need to be conducted with vaccines that expand cell populations capable of producing other bnAbs with greater potency than VRC01, and which target parts of the HIV envelope protein other than the CD4 binding site. This may be more difficult, as other bnAbs derive from cells with less specific genetic signatures.
The main challenge, however, it that this is only the first step in this vaccine concept. The next step will be to sequentially inoculate people with a series of vaccines containing epitopes based ever more closely on actual HIV virus env proteins.
It’s problematic as to how this can be done. A vaccine containing numerous HIV epitopes might have to have high volume, and also runs the risk that it could overwhelm the immune system and produce a massive but non-specific reaction that might not only be unsafe but would also not work against HIV. The alternative is to give people a long sequence of vaccines, aiming to ‘shepherd’ the immune system into producing bnAbs, but this would be challenging logistically and clinically.
One possible answer is that the next-stage germline targeting vaccines, in trials G002 and so on, will use mRNA-based vaccines, not virus-like particles. These will tell cells to make their own particles, not inject then direct. This implies that a number of different instructions could be packaged into a smaller dose of vaccine and also that the manufacturing of varied vaccine jabs should be easier. The G003 trial is taking place in Rwanda and South Africa, which may answer the question of whether a vaccine designed to elicit VRC01-works as well in African populations.
Essentially what the Scripps researchers have done is to create a significant population of cells, at least potentially capable of making one particular type of bnAb, that was insignificant before. The idea, to use a metaphor, in the hare-versus-tortoise race of HIV versus the immune system, is to equip the tortoise with a jet pack so the hare finds it waiting at the finish line.
Schief and colleagues comment that they have achieve “an unprecedented level of control over the specificity of immune responses” that may “herald new era of precision vaccine design for HIV and other pathogens”. In other words, while it may still be challenging to turn this priming vaccine into an entire, effective prime-and-boost course of vaccines, the scientific approach it uses has huge implications for more sophisticated vaccine design generally.
References
Leggat DJ, Cohen KW, Schief WR et al. Vaccination induces HIV broadly neutralising antibody precursors in humans. Science, 378(6623): pre-print publication, 2 December 2022.
www.doi.org/10.1126/science.add6502.
Moore PL. Triggering rare HIV antibodies by vaccination. Science 378 (6623): 949-950, 2 December 2022.
www.doi.org/10.1126/science.adf3722.