Action Region will wrap only the input Field and not the complete pageBlockSectionItem. Place pageBlockSectionItem tags inside a pageBlockSection, which in turn should be in a pageBlock. This will provide the correct formatting.

<apex:page standardController="Campaign" showHeader="true" >
	<apex:form>
		<apex:outputPanel id="op1">
			<apex:pageBlock>
				<apex:pageBlockSection columns="2" title="Information">
					<apex:pageBlockSectionItem>
						<apex:actionRegion>
							<apex:inputField label="{!$ObjectType.Campaign.fields.Category__c.Label}" value="{!Campaign.Category__c}"> 
								<apex:actionSupport event="onchange" rerender="op1" />
							</apex:inputField>
						</apex:actionRegion>
					</apex:pageBlockSectionItem>
				</apex:pageBlockSection>
			</apex:pageBlock> 
		</apex:outputPanel>
	</apex:form>
</apex:page>