139During B cell development, the near random nature of the VDJ recombination process leads to the unavoidable production of self-reactive antibodies. In fact, studies in humans have demonstrated that many if not most newly generated B cells are autoreactive ( 1 ). Extensive studies of self-tolerant B cells in transgenic mouse models have revealed the complicated systems of B cell selection used to avoid autoimmunity. Current models suggest that B cells expressing a transgenic surface Ig that binds DNA or protein autoantigens fi rst attempt to alter the B cell receptor (BCR) by further variable gene rearrangement using " receptor editing " ( 2, 3 ). If receptor editing is unsuccessful, then the offending B cell may be eliminated by clonal deletion ( 4, 5 ) or it may enter maturity but with reduced or altered function so that it no longer reacts to the self-antigens, which is referred to as clonal anergy ( 6 -8 ). In this paper, we describe a human B cell population that is anergic. Clonal anergy was fi rst conceived by Nossal and Pike in 1980 ( 6 ) to explain why injection of neonatal mice with high dosages of an antigen induced deletion of the specifi c B cells, whereas lesser dosages allowed retention of the specifi c B cells, but the cells were incapable of becoming antibody-secreting cells. In 1988, Goodnow et al. ( 8 )

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Current influenza virus vaccines rely upon the accurate prediction of circulating virus strains months in advance of the actual influenza season in order to allow time for vaccine manufacture. Unfortunately, mismatches occur frequently, and even when perfect matches are achieved, suboptimal vaccine efficacy leaves several high-risk populations vulnerable to infection. However, the recent discovery of broadly neutralizing antibodies that target the hemagglutinin (HA) stalk domain has renewed hope that the development of "universal" influenza virus vaccines may be within reach. Here, we examine the functions of influenza A virus hemagglutinin stalk-binding antibodies in an endogenous setting, i.e., as polyclonal preparations isolated from human sera. Relative to monoclonal antibodies that bind to the HA head domain, the neutralization potency of monoclonal stalk-binding antibodies was vastly inferior in vitro but was enhanced by several orders of magnitude in the polyclonal context. Furthermore, we demonstrated a surprising enhancement in IgA-mediated HA stalk neutralization relative to that achieved by antibodies of IgG isotypes. Mechanistically, this could be explained in two ways. Identical variable regions consistently neutralized virus more potently when in an IgA backbone compared to an IgG backbone. In addition, HA-specific memory B cells isolated from human peripheral blood were more likely to be stalk specific when secreting antibodies of IgA isotypes compared to those secreting IgG. Taken together, our data provide strong evidence that HA stalk-binding antibodies perform optimally when in a polyclonal context and that the targeted elicitation of HA stalk-specific IgA should be an important consideration during "universal" influenza virus vaccine design. IMPORTANCEInfluenza viruses remain one of the most worrisome global public health threats due to their capacity to cause pandemics. While seasonal vaccines fail to protect against the emergence of pandemic strains, a new class of broadly neutralizing antibodies has been recently discovered and may be the key to developing a "universal" influenza virus vaccine. While much has been learned about the biology of these antibodies, most studies have focused only on monoclonal antibodies of IgG subtypes. However, the study of monoclonal antibodies often fails to capture the complexity of antibody functions that occur during natural polyclonal responses. Here, we provide the first detailed analyses of the biological activity of these antibodies in polyclonal contexts, comparing both IgG and IgA isotypes isolated from human donors. The striking differences observed in the functional properties of broadly neutralizing antibodies in polyclonal contexts will be essential for guiding design of "universal" influenza virus vaccines and therapeutics. I nfluenza A viruses (IAVs) remain one of the most pressing global public health concerns due to their widespread distribution, rapid evolution, and potential for reassortment (1). These traits contribute to the abil...

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