Controlled human infection with Plasmodium falciparum-infected mosquito bites elicits antibodies against mosquito salivary protein SG1L3

This study identifies human monoclonal antibodies generated by *Plasmodium falciparum*-infected mosquito bites that target the mosquito salivary protein SG1L3 bound to sporozoites, revealing it as a potential serological marker for mosquito exposure rather than a viable target for blocking malaria infection.

The Gist

The Big Picture: A Case of Mistaken Identity (Sort of)

Imagine the malaria parasite (Plasmodium falciparum) is a master thief trying to break into a house (your liver). To get in, the thief doesn't just walk through the front door; they hitch a ride on a delivery truck (the mosquito).

When the mosquito bites you, it injects the thief along with its own "saliva" (which acts like a lubricant to stop your blood from clotting). For decades, scientists have been trying to build a security system (a vaccine) that recognizes the thief specifically. They found a very obvious uniform the thief wears called CSP (Circumsporozoite Protein). Current vaccines target this uniform.

However, scientists noticed that even when they blocked the thief's uniform, the security system still seemed to have some "muscle" left over. They wondered: What else is the body fighting?

The Discovery: Finding the "Delivery Truck" Uniform

In this study, the researchers took a volunteer who had been bitten by malaria-infected mosquitoes (under strict medical supervision) and looked at their immune cells. They were hunting for antibodies (the security guards) that could stop the thief without targeting the thief's main uniform (CSP).

They found two special "guards" (monoclonal antibodies). But here is the twist: These guards weren't looking at the thief at all.

Instead, they were looking at the delivery truck's uniform.

The researchers discovered that these antibodies were actually targeting a protein called SG1L3. This protein comes from the mosquito's saliva, not the malaria parasite. When the mosquito bites, it leaves a little bit of this saliva protein stuck to the parasite, like a sticker on a package. The human body made antibodies against the sticker, not the package inside.

The Experiment: Does the Sticker Matter?

The team asked a crucial question: If we have antibodies against the mosquito's saliva sticker, does that stop the thief from breaking into the house?

To test this, they ran simulations:

1. The Test: They mixed the parasite with these "anti-sticker" antibodies.
2. The Result: The thief still got into the liver cells perfectly fine. The antibodies against the mosquito saliva did nothing to stop the infection.

It's like having a security guard who is excellent at recognizing the delivery truck's logo, but once the truck drops off the package, the guard just stands there and watches the thief walk away. The guard is looking at the wrong thing to stop the crime.

The Silver Lining: A New "Mosquito Meter"

So, if these antibodies don't stop malaria, why does this matter?

The researchers found something very useful. They looked at people living in Burkina Faso, an area with lots of mosquitoes. They found that:

• Children with few mosquito bites had low levels of these antibodies.
• Older people with many years of mosquito bites had high levels of these antibodies.

The Analogy: Think of these antibodies like a mosquito "fingerprint" on your skin.
If you get bitten a lot, your body builds up a lot of "anti-mosquito-saliva" antibodies. If you get bitten less, those levels drop.

Because these antibodies don't fade away quickly and grow stronger with more exposure, they are perfect for tracking. Scientists could use a simple blood test to see how many mosquitoes a community is actually being bitten by. This is a huge deal for public health! It helps governments know if their mosquito control efforts (like spraying or nets) are actually working, even if the mosquitoes are too small to count directly.

Summary

1. The Hunt: Scientists looked for new ways to stop malaria by finding antibodies that ignore the parasite's main uniform.
2. The Surprise: They found antibodies that target the mosquito's saliva (SG1L3) instead of the parasite.
3. The Bad News: These antibodies don't stop the malaria infection. They are "distracted" by the mosquito's saliva.
4. The Good News: These antibodies are a fantastic tracker. They act like a "mosquito exposure meter," helping scientists measure how much people are being bitten in different areas to improve malaria prevention strategies.

In short: We found a security guard that can't stop the thief, but is excellent at counting how many delivery trucks are visiting the neighborhood.

Technical Summary

1. Problem Statement

Malaria remains a significant global health burden, with current vaccines (RTS,S and R21) targeting the Plasmodium falciparum circumsporozoite protein (CSP) showing only partial efficacy. While antibodies against CSP are well-characterized, the immune response to non-CSP antigens on the sporozoite surface remains poorly understood. Previous studies suggested that polyclonal antibodies from volunteers immunized with whole sporozoites could block liver-stage infection even after CSP-specific antibodies were removed, implying the existence of functional non-CSP targets. However, the specific molecular targets of these protective non-CSP antibodies had not been identified. Furthermore, it was unclear whether antibodies against mosquito salivary proteins (co-inoculated with parasites) could contribute to protection or serve as markers of exposure.

2. Methodology

The study employed a multi-step approach combining controlled human infection, single B-cell sorting, and functional assays:

• Study Cohort: Researchers utilized samples from a Dutch volunteer who underwent a Controlled Human Malaria Infection (CHMI) via bites from Anopheles stephensi mosquitoes infected with P. falciparum (NF54 strain) while on chemoprophylaxis. This volunteer exhibited strong non-CSP-mediated inhibition of hepatocyte invasion.
• Target-Agnostic B-Cell Sorting:
  • Single memory B cells (CD19+ IgM- IgA- IgD-) were sorted from the volunteer's peripheral blood mononuclear cells (PBMCs).
  • Cells were stimulated for two weeks, and supernatants were screened for reactivity against P. falciparum sporozoite lysate.
  • Reactivity against recombinant CSP was used to exclude CSP-specific clones.
• Monoclonal Antibody (mAb) Generation: Heavy and light chain genes were amplified via nested PCR from 264 reactive clones, cloned into IgG1 expression vectors, and expressed in HEK293T cells.
• Target Identification:
  • Immunoprecipitation (IP) & Mass Spectrometry (MS): mAbs were used to IP proteins from uninfected mosquito salivary gland extracts, followed by LC-MS/MS to identify the bound proteins.
  • Validation: Recombinant proteins (SG1L3 and other SG1 family members) were expressed in S2 or HEK293T cells. Binding was confirmed via ELISA, Western Blot, and Flow Cytometry.
  • Localization: Immunofluorescence microscopy was used to determine the location of SG1L3 in mosquito salivary glands and on sporozoites.
• Functional Assays:
  • In Vitro Inhibition: mAbs and polyclonal IgGs (from the human volunteer and SG1L3-immunized rabbits) were tested for their ability to inhibit sporozoite traversal and hepatocyte invasion using HC-04 liver cells.
  • Depletion Studies: SG1L3-specific antibodies were affinity-depleted from the volunteer's polyclonal IgG pool to assess their contribution to the observed inhibition activity.
• Epidemiological Analysis: ELISA was performed on plasma samples from a cohort in Burkina Faso (Sapone health district) to assess natural antibody responses to SG1L3 across different age groups.

3. Key Contributions

• First Identification of Human Anti-Sporozoite mAbs Against Non-CSP Targets: The study successfully isolated the first human monoclonal antibodies that recognize non-CSP antigens on the sporozoite surface.
• Discovery of SG1L3 as a Sporozoite-Bound Antigen: The researchers identified that the target of these mAbs is SG1L3, a salivary protein from the Anopheles stephensi mosquito, which binds to the surface of P. falciparum sporozoites.
• Differentiation of Protective vs. Non-Protective Antibodies: The study demonstrated that while SG1L3 is a major antigen recognized by the immune system, antibodies against it do not mediate liver-stage protection.
• Potential Serological Marker: The study provides evidence that SG1L3 antibodies accumulate with age in endemic areas, suggesting its utility as a marker for cumulative mosquito exposure.

4. Key Results

• Antibody Isolation: Two mAbs (16O7 and 4G16) were isolated. Both recognized a ~40-45 kDa protein in sporozoite lysate but did not bind recombinant CSP.
• Target Identification: Mass spectrometry identified SG1L3 as the primary target. Both mAbs bound specifically to recombinant SG1L3 but not to other SG1 family members (SG1, SG1a, saglin).
• Surface Binding:
  • SG1L3 was localized to the median lobe of the mosquito salivary glands.
  • SG1L3 binds to the surface of P. falciparum sporozoites (confirmed by flow cytometry and immunofluorescence).
  • Recombinant SG1L3 protein binds to sporozoites but not to gametes.
• Lack of Protective Function:
  • Neither mAb 16O7 nor 4G16 inhibited sporozoite traversal or hepatocyte invasion in vitro.
  • Depletion of SG1L3-specific antibodies from the volunteer's polyclonal IgG pool did not reduce the inhibition of liver-stage infection, proving that SG1L3 antibodies were not responsible for the observed protection.
  • Rabbit immunization with recombinant SG1L3 generated high-titer antibodies that recognized sporozoites but failed to inhibit invasion.
• Natural Exposure: In the Burkina Faso cohort, SG1L3 antibody prevalence increased with age (9.5% in <5 years, 48.8% in >15 years), indicating that natural mosquito bites induce these antibodies and that they do not induce immune tolerance (unlike some other salivary proteins like SG6).

5. Significance

• Vaccine Development: The findings clarify that while non-CSP antibodies exist, the specific target identified (SG1L3) is not a viable candidate for a transmission-blocking or liver-stage vaccine, as the antibodies do not neutralize the parasite. This refocuses the search for functional non-CSP targets.
• Vector-Host Interaction: The study confirms that mosquito salivary proteins can bind to sporozoites during transmission. While SG1L3 itself may not be protective, understanding these interactions is crucial for modeling infection dynamics.
• Epidemiological Tool: SG1L3 emerges as a promising serological marker for monitoring mosquito exposure. Unlike SG6 (which induces tolerance) or cE5 (which induces long-lived IgG), SG1L3 antibodies correlate with cumulative exposure and age, making them potentially useful for evaluating the success of vector control interventions in endemic regions.
• Methodological Advance: The study validates the use of target-agnostic single B-cell sorting combined with IP-MS as a powerful tool for uncovering novel antigen targets in complex biological systems like malaria infection.